They are not hard to find. Every few days brings a fresh tale of feral youths meting out random acts of violence with unfathomable intensity. Apart from the shocking brutality, the speed with which a seemingly trivial argument or confrontation can assume murderous proportions, the stories have a common theme: the perpetrators of the violence, often in their very young teens, were high on 'skunk' at the time.

The teenagers who killed Garry Newlove, the 47-year-old father of three in Cheshire? The attack came after they had binged on alcohol and skunk.

Last month three youths were found guilty of kicking to death Mark Witherall, 47, after he found them burgling his house in Whitstable, Kent. The three were intoxicated by a ferocious cocktail of alcopops and cannabis. The judge said the three had 'acted as hyenas'.

And last week the mother of Sophie Lancaster, the 20-year-old goth murdered by two binge-drinking teenagers, claimed the rise of skunk was now one of the biggest causes of problems among young people. 'It's so much stronger now than normal cannabis and young people are smoking it from 9am and thinking it's OK,' said Sylvia Lancaster. 'I have worked with young people over a number of years and I believe that one of the biggest issues facing us is skunk.'

Suddenly, skunk - a high-strength herbal strain of cannabis - is showing the darker side of a drug that was once considered to be relatively benign. Concerns about its links with mental illness and its ability to act as a 'gateway drug', leading users into addiction, have prompted a sea change in popular opinion about cannabis.

It's a far cry from the Sixties, when cannabis - chiefly marijuana, or 'grass' - promised to open an entire generation's mind to new possibilities. As Paul McCartney observed to one of his biographers: 'We'd met people like Dylan and we got into pot, like a lot of people from our generation. And I suppose in our way we thought this was a little more grown-up than perhaps the Scotch and Coke we'd been into before then... so once pot was established as part of the curriculum you started to get a bit more surreal material coming from us, a bit more abstract stuff.'

The Beatles followed earlier converts such as America's beat poets - people like Jack Kerouac, Allen Ginsberg and William Burroughs, who extolled the virtues of cannabis and helped consolidate the drug's image as a magic door through which users could access a world of new experiences. During the Vietnam war, dope-smoking assumed a political dimension, a sign that its users were at odds with a US government fighting an unpopular war. Cannabis soon became a major component of the West Coast counter-culture, a facilitator of free love and a return to an almost sybaritic era.

But now the hippie dream of peace and love has turned into the nightmare of A Clockwork Orange, in which drug-fuelled youths go on a violent rampage.

'The caricature of cannabis has for years equated it with herbal tea and hippies,' says Ben Lynam, of the UK Drug Policy Commission. 'Some people still believe that, but they are now very much among the minority.'

Prime Minister Gordon Brown is not one of that minority. This week he is expected to reject the opinion of the government's own Advisory Council on the Misuse of Drugs (ACMD), and signal that cannabis should be reclassified from a class C to a class B drug, a spectacular U-turn from just two years ago and the first time ministers have ignored their experts since 1971.

It means those caught in possession of cannabis could face prison sentences of up to five years, compared with two now. But the change, for the majority of users, is likely to be cosmetic. The police have signalled that they will still continue a policy of 'confiscate and warn', although persistent offenders will face tougher penalties which experts believe will see more ending up in prison. Last year police warnings on cannabis rose 20 per cent to 120,000, suggesting the new approach is proving popular with officers on the streets, as it frees them from the bureaucracy associated with making arrests.

Brown's supporters insist the reclassification will send a signal to society and in particular to young people. 'We need to send a message out that drug-taking is wrong,' says Vernon Coaker, the Home Office minister. 'It's an illegal substance, so it's about ensuring that we keep that message strong and powerful.'

But the shift by some is seen as Brown exerting his authority as a conviction politician, obeying his own puritanical code rather than listening to the experts. Few in the field believe the move is justified or indeed will have any effect. In 2006, the House of Commons Science and Technology Committee found 'no solid evidence' that classification had a deterrent effect on consumption. Only 3 per cent of people polled by the mental health charity Rethink said a change in classification would deter them from smoking cannabis.

'The cost of reclassification will be more than £1m,' says Paul Corry, director of public affairs at Rethink. 'Redrafting legislation, telling the public about it and retraining police is an expensive business. But we know reclassification won't reduce the numbers using cannabis, so it will be a waste of money.'

Something strange is happening on Britain's streets: drugs have become cheaper. A survey by the charity DrugScope, based on interviews with street dealers, reveals the price of a gram of heroin dropped from £46 in 2006 to £43 last year. An ecstasy pill cost £2.40 last year compared with £3 the year before. Ketamine and and crystal powder also saw price falls while the price of cocaine remained stable. The price of cannabis, however, rose over the same period. While an ounce of 'normal' herbal cannabis would set you back £70 in 2006, last year it would cost you £87. And the price of an ounce of the stronger strains has risen to £134, up from £121.

Cannabis is now a big black-market business in Britain. While heroin is imported from the east, cocaine from South America and ecstasy from the Netherlands, much of the cannabis crop is homegrown. Sir Stephen Lander, the head of the Serious and Organised Crime Agency warned earlier this year that large-scale cannabis factories - producing high-strength strains of the drug and run by Vietnamese and Chinese criminals - are appearing across the country.

Charities working with immigrant communities claim that in many cases the factories rely on smuggled child labour to maintain the plants. What was a cottage industry has become an industrialised cultivation. Growers now use state-of-the art hydroponic systems to ensure bumper crops.

'There are wide areas of the country where this is being grown commercially,' Lander said. 'It's not as though people are growing it in a couple of pots on their window sills.'

Soaring demand for the stronger strains of cannabis is reflected by police seizures. The Home Office has been quietly studying the results of a survey conducted among police forces across the UK. Early findings suggest sinsemilla - the potent herbal leafy variety of cannabis made from dried seedless female plants, of which skunk is just one of about 100 strains - accounts for 80 per cent of all cannabis seizures. Meanwhile, seizures of cannabis resin - which back at the turn of the millennium far exceeded the number of seizures of sinsemilla - have dropped by more than a third.

This is the trend that has alarmed many health experts. Sinsemilla contains far higher amounts of the psychoactive substance tetrahydrocannabinol (THC), the magic ingredient that brings euphoria. Unpublished studies suggest the THC content of the strongest varieties of herbal cannabis has doubled over the past 10 years, from seven to 14 per cent. Some strains of herbal cannabis - so-called 'super skunk' - now contain as much as 46 per cent THC. At the same time, THC levels in resin have been falling, down to an average 3.3 per cent last year.

The crucial issue dividing politicians and mental-health experts, though, is whether this polarisation of cannabis is having a deleterious effect on the nation's mental health. Studies suggest the new hybrid strains of cannabis such as skunk, which are believed to have originated in the West Coast of the US in the Seventies, contain extremely low levels of the anti-psychotic agent, cannabidiol, leading to claims they may be more harmful.

A study published in the British Medical Journal found those using cannabis before the age of 15 are four times as likely to develop psychotic illness by 26. A Lancet study in 2007 estimated that 14 per cent of 15- to 34-year-olds affected by schizophrenia are ill because of heavy cannabis use. And recent analysis of 35 major studies concluded that cannabis use increased the risk of psychotic illness later in life by approximately 40 per cent and by up to 200 per cent among heavy users.

Many experts in mental health say they now have more than enough evidence to understand that cannabis is not the safe drug of popular myth.

'We have been campaigning for many years about the links between cannabis and psychiatric illness, and highlighting evidence that the drug may not only precipitate psychotic breakdown but cause long-term mental damage,' says Marjorie Wallace, chief executive of the mental health charity Sane.

'The front-line experience of organisations such as ours is that use of the drug can cause harm, not only to young people but to their families, making the outcomes worse for those with mental illness and robbing young people of their motivation and future.'

Wallace accepts some of the research appears to be conflicting, but says we ignore the worst-case scenario at our peril. 'We consider that until we conduct more studies to establish the effects of the drug on developing brains and minds, the strongest signals must be given that it can be disproportionately dangerous to those who may be at most risk,' Wallace says.

The problem for the anti-cannabis camp, however, is that cases of psychosis have actually been falling. Research presented to the ACMD - and considered instrumental in persuading it that there is no need to reclassify cannabis - have shown incidents of schizophrenia have declined between 1996 and 2005.

And claims that Britain is in the grip of a cannabis epidemic also look flawed. According to government surveys, reported use of cannabis among 11- to 15-year-olds dropped from 13.4 per cent in 2001 to 10.1 per cent last year. Over the same period, reported use of cannabis among 16- to 24-year-olds slid from 27.3 per cent to 20.9 per cent.

Significantly, the decline in reported cannabis use continued to fall after the drug was reclassified from class B to class C. 'The gentle decline is something we have seen in other countries, too,' said Ben Lynam, of the UKDPC. 'It's difficult to say what is driving this. It may be that people are switching to something else, like binge-drinking.'

Likewise, the number of children who believe it is 'OK' to do cannabis has dropped dramatically - from 17 per cent in 2003 to 9 per cent in 2006, according to the Department of Health.

And yet you could be forgiven for thinking otherwise, given the lurid headlines about a skunk-induced orgy of violence. 'There is a media generalisation about cannabis,' Lynam said. 'You read about people going on an all-night bender on cider but the focus in the reports is on the fact they had a spliff. That's not to say some people don't achieve psychosis through a spliff - in some isolated cases that will be the case. But it all helps to create some ridiculous image of cannabis as the evil weed.'

Talk to practically any expert in the field and though they may disagree on the relative dangers of cannabis use, they will agree on one thing: the debate about reclassification has dangerously stymied the wider debate about drugs. Instead, experts complain the focus on whether cannabis should be class B or C has blinded the politicians to the real issue - whether classification works at all.

'It is important that policy is grounded in evidence,' says Martin Barnes, chief executive of DrugScope and a member of the ACMD. 'But the concern is now that policy is being driven by political considerations and headlines.'

The UKDPC will issue a briefing paper to politicians next week that quotes Professor Colin Blakemore of the Science and Technology Committee. 'If it took so much effort to consider one particular drug and whether it should be placed on one side or other of a boundary, does it not imply that the entire mechanism for classifying requires a new look?'

A small but increasingly vocal band of experts would go further and see all drugs legalised. They say the experience of the Netherlands, where people are allowed to smoke cannabis in licensed bars, suggests legalisation does not increase usage.

And they claim it would break the links between organised crime and drugs. As a 2004 unpublished Home Office briefing to Tony Blair suggested: 'There is a strong argument that prohibition has caused or created many of the problems associated with the use or misuse of drugs. One option for the future would be to regulate drugs differently, through either over-the-counter sales, licensed sales or doctor's prescription.'

But instead of the wider debate many had hoped for, the government seems to be retrenching. In addition to getting tougher on cannabis, its long-awaited 10-year drugs strategy is, according to experts, simply a replica of the previous strategy.

'This is all smoke and mirrors,' said Danny Kushlick, director of Transform Drug Policy Foundation, the drugs think-tank that advocates legalisation. 'It is time for Brown and Cameron to stop the Dutch auction on who can be toughest on cannabis and begin a genuine exploration of alternatives to prohibition which costs UK taxpayers £2 billion to enforce and has created a drugs market worth £5bn.'

That it has come to this, a furious debate about the merits of reclassification of a single drug, is a singular failure of the government, Kushlick believes. He points out that five years ago, the Home Office select committee went as far as to debate openly the merits of opening up a comprehensive discussion on alternatives to prohibition. A then relatively obscure Tory MP, David Cameron, backed the move while a shadow minister, Alan Duncan, supported the idea of full legalisation.

But instead, experts believe the renewed focus on cannabis has now turned the clock back, with worrying implications. 'Resources could be put to much better use educating young people and the public about the physical and mental health risks associated with cannabis - we know education, and health warnings are a cost-effective way to get results,' said Paul Corry of Rethink. 'We urge Gordon Brown to consider the facts and do the right thing with tax payers' money - don't waste time tinkering around with classification - invest in drugs education.'

A decision to reclassify cannabis this week will also have repercussions across the criminal justice system and end up hitting a disproportionate number of young people, according to those working at the frontline of the drugs war.

'If you reclassify cannabis, that will put pressure on the police to bring more charges,' said Harry Fletcher, of the probation officers' union, Napo. 'That will put more pressure on the Crown Prosecution Service and more pressure on the courts. You will have more people going down. You will have more people receiving sentences because they are addicted, not because they are core criminals.'

Instead of tougher penalties, most experts would prefer the government to spend its money trying to understand the more potent forms of cannabis that are emerging on Britain's streets. Although the government will this week bolster its argument for the reclassification of cannabis by publishing evidence that skunk is now the strain of choice among cannabis smokers and that the average spliff contains 16 per cent THC, the jury is still out on what this means.

There is evidence that users are moderating their consumption, aware of the drug's increased potency. Equally there is evidence cannabis can have some positive health effects for cancer sufferers. But despite the fact that cannabis consumption can be traced back to the Neolithic age, it is still a little understood drug.

Britain's increasingly hardline approach seems at odds with many other countries. There is a worldwide trend towards decriminalisation of cannabis. Holland, Switzerland, Spain, Italy, Portugal, western Australia, the Russian federation, and some Canadian and US states have moved to civil penalties for drug possession. Barack Obama - who said of his youthful cannabis use, 'I inhaled. That was the point' - has pledged to decriminalise cannabis if he becomes US president.

But in Britain experts fear the cannabis debate is simply the start of a series of skirmishes between Gordon Brown's government and its advisers. The Observer understands that the ACMD is soon to start looking at whether ecstasy should be reclassified from a class A drug to a class B drug. The evidence base seems to support the move, but it seems incomprehensible, given the current febrile climate, that the government will sanction the move. Instead it will once again defy the experts, a move that will trigger further accusations that the issue is simply a political football.

The great irony is that the debate around cannabis - the drug of choice of the flower-power generation, the narcotic which in the Sixties and Seventies promised to open people's minds and spark a cultural revolution - is now restricting understanding. It is not just Britain's youth that may have been damaged by skunk.

What do you think? review@observer.co.uk

The user: A young man's style

William is 20. He began smoking cannabis at the age of 14, at his public school in south London. 'Cannabis runs through the whole class system,' he says. 'There was a large group of people smoking regularly by the time we were 15, and there was a boy, a good friend, selling it in class. By the time I was picking up regularly, when I was 16, it was all skunk.'

Why did he start smoking? 'To get a buzz, to have a giggle and for recreation. Every party I went to revolved around skunk. When you become a teenager, friends become the most important thing in your life. I had always been the "good first son" and this was something my parents knew nothing about.'

At the height of his habit, William was smoking £10-worth a day, around 12g a week. He says it was an important factor in him losing his family.

'It gave me a shorter fuse; my morals seemed to leave me. We would argue all the time over the most trivial of things, but I knew it was over my smoking.'

'He became a stranger in our midst,' William's mother, Debra Bell, says. 'Aged 14 he was pleasant, optimistic, thoughtful. He loved reading. Then he became depressed, upset with himself and very upset with us.'

According to Bell, by the time William was 16, he was refusing to go to school, would get verbally abusive and violent.

'When he started to steal from us and his brothers, that was just a huge betrayal,' Bell recalls. 'Money, CD players and PS2 games would go missing: it was like living with a burglar. We had to put our cash, jewellery and cheque books in a safe. Any sense of respect had gone on both sides.'

When William was 16, they asked him to leave home and set him up in a flat on his own. 'And all of the time there was the sense of what's going on, is it just us?'

Bell began keeping a diary of her experiences. Last year she published them on a website (talkingaboutcannabis.com) and received hundreds of emails from parents who had been through a similar experience. Together they formed a pressure group called Talking About Cannabis. 'There wasn't an understanding about how strong skunk was and what it would mean in these crucial teenage years,' Bell says. 'If you drop out or screw up at school you're leaving yourself a mountain to climb.'

William scraped through his GCSEs but gave up on A-levels. He now works in a pub and has got himself a flat in south London. He has reduced his cannabis smoking considerably, and tries to steer clear of skunk. His mother's work has made him thoughtful about the effects of cannabis, and he has canvassed his friends' opinions.

'Surprisingly, everyone accepts that it has had a negative, depressing effect on them but it varies massively from person to person. One friend had a smoke the other day and within minutes had a paranoid episode; he saw spiders crawling out of the lamp. But he felt fine the next day.' Interview by Tom Templeton

Cannabis sativa: Facts and figures

The statistics

· 2.7 million Britons aged between 16 and 59 say they smoked cannabis in the last month, making it the UK's second favourite intoxicant after alcohol.

· In 1995, the average number of plants seized in police raids on cannabis factories was 15. By 2007, this had increased to 900 which would give an annual yield to growers of £300,000.

· Sinsemilla - the potent variety of cannabis, of which skunk is just one of about 100 strains - accounts for 80 per cent of all cannabis seizures.

· Levels of THC , the major psychoactive component of cannabis, are around 3.3 per cent in resin. THC levels in some varieties of skunk can be as high as 46 per cent

· A British Medical Journal study found those using cannabis before the age of 15 are four times more likely to develop psychotic illness by 26.

· More than 13,000 adults and 11,000 children received treatment for the effects of cannabis abuse in the UK in 2006-7.

· In 2007, police warnings on cannabis rose 20 per cent to 120,000.

· Use of cannabis among 11- to 15-year-olds dropped from 13.4 per cent in 2001 to 10.1 per cent last year.

· The Netherlands, Switzerland, Spain, Italy, Portugal, the Russian Federation, and some Australian, Canadian and US states have moved to civil penalties for drug possession.

The science

· Sinsemilla is produced through selective breeding. Female cannabis plants are grown without a male so that the plant produces more of the crystal-coated filaments that cause the high.

· Cannabis smoke contains 483 chemicals, of which 66 are cannabinoids, unique to the drug

· The effects of the drug are felt as the cannabinoids fit on to thousands of receptor proteins in the brain. This causes a series of commands to be relayed to the central nervous system and the body: lowering body temperature, impairing memory and movement control and releasing dopamine to the brain.

· The effects of THC take hold within seconds of inhaling cannabis smoke and are at their strongest after a few minutes.These include fits of laughter, and enhanced audio and visual sensation. Ally Carnwath

The psychiatrist: Reefer madness

Halfway down south London's Landor Road - notorious in recent years for teen shootings and police raids - sits Lambeth Hospital. Dr Zerrin Atakan worked as a consultant psychiatrist at the intensive care unit here for 13 years.

Throughout the Eighties and Nineties patients came and went and came again through Lambeth Hospital's mental health wards. To end up in Atakan's intensive care ward they would be in an extreme state, and were often violent. Atakan began to observe that most of them smoked cannabis.

Psychiatric intensive care wards are locked wards but, if a patient's mental state has stabilised, after a few weeks they are allowed out to shop for a newspaper, to see how things are going.

'Some would celebrate their freedom with a joint,' Atakan recalled, 'and psychologically they'd be straight back to square one.

'It became such a regular situation that we'd guess it straight away, and then it would be confirmed by urine analysis.

'What we know for a fact from epidemiological studies is that if you smoke long enough and strong enough, there is an increased risk of psychotic outcomes,' says Atakan. Two in every 100 frequent cannabis users will suffer schizophrenia.

'We also now know that if you happen to have psychosis and carry on smoking, your progress outcomes are worsened.

'But to be convinced that there is a direct link we have to have biological evidence.'

To her amazement Atakan found that there were hardly any studies about the links between psychosis and cannabis use. It took her and collaborator Professor Philip McGuire 15 years to get funding and cut through the red tape to run a study. 'It wasn't considered a sexy drug,' she recalls, 'and medics can't buy drugs on street corners.' But finally, using an oral cannabis medication from a government pharmacy, they are beginning to get answers.

One test Atakan ran was a response inhibition task to measure how well people stop themselves from doing something. When people stop themselves the cortex area of the brain fires up, and in people with schizophrenia, cortex function is reduced. This 'attenuation' of activity is linked with paranoia or suspiciousness. In other words we are hardwired to feel paranoid or suspicious, and our cortex has to work to keep these feelings in check.

A group of volunteers were shown a series of images of arrows. They were asked to press a button if the arrows were horizontal - whether they pointed left or right - but to stop themselves if the arrows were at an oblique angle. The group undertook the test twice, once following an oral dose of THC and the second time having been given a placebo.

'Activity in the cortex was quite significantly attenuated by THC [the main psychoactive substance found in the cannabis plant] compared to the placebo,' says Atakan. In other words, THC made them bad at stopping themselves.

Atakan's team also measured the group's psychotic state - their anxiety levels, sense of alienation, intoxication and general wellbeing. The psychotic symptoms appeared more pronounced the less well the subjects were able to 'inhibit' themselves, in line with THC consumption.

A current Institute of Psychiatry study has healthy adults injected intravenously with THC and observed. It has found that THC introduces psychotic symptoms. Paranoia is increased, as is misinterpreting the environment. Reasoning is impaired and there is a disconnection between thinking about moving and moving, and thinking about speaking and speaking.

It seems incontrovertible that THC pushes people up the psychotic scale.

'But cannabis doesn't cause psychosis in everyone,' says Atakan. 'There are young people who smoke every day but just get mild paranoia, and when they stop smoking, that goes away too.'

Why do some people return to normal and others stay down in the dark?

'We are at a premature stage in our knowledge. Especially in genetic terms. Some people think there may be a gene or combination of genes that makes you vulnerable. I think there may be protective genes that stop certain people becoming too psychotic.' TT

The dealer: Growing demands

Carl works in the film industry. He is in his early thirties and has been a chronic cannabis smoker since he was 18. About five years ago he 'found himself' becoming a dealer.

'I was holding pretty regularly, and friends would smoke a lot of my stuff. The more I smoked and gave to my friends, the more it made sense to buy a nine bar and sit on it for a month or so. I make enough to fund my own use.'

Suppliers sell skunk in 6kg loads which are split into 1kg bricks, then quartered into 'nine bars', which then provide nine single-ounce bags which are then split into eighths. The drug often changes hands at each stage.

'Once I've got this stuff it's been across the country three times,' Carl says.

The buzzer sounds and Carl lets in two of his friends, or friends of friends - skinny, pale men in their late twenties. Len hands Carl a £20 note and receives an eighth of skunk in return. We adjourn to a living room adorned with Kubrick film posters and Dalí prints. Pete, a heavily side-whiskered musician, gets rolling: crumbling the dried skunk buds on to a net of three cigarette papers. There is no typical 'smoke-up' or 'blaze' because there is no typical cannabis smoker.

Cannabis is Britain's second favourite intoxicant after alcohol. Doctors, down-and-outs, lawyers, criminals, police, journalists, celebrities, pen-pushers, thugs, peaceniks and traffic wardens are all among the 2.7 million or so Britons aged 16-59 who report having smoked in the last month. Anecdotal evidence suggests the demand for skunk grew in the late Nineties as middle-class smokers became wary of smoking resin. 'Soap bar' was hash resin that tripled in weight on its journey from Morocco to Britain through adulteration.

'I knew a guy who worked in a cannabis factory in Spain,' Carl says. 'They'd melt the resin down, throw all kinds of shit in it - coffee, aspirin, asphalt - and then add boot polish or paint for the colour.' TT