Against the steady hum of an extractor fan, the handheld video camera pans over a small forest of green plants. The unmistakable serrated leaves of cannabis flutter as gentle gusts of warm air from a heating unit waft over them.

The man who uploaded the video on to YouTube can barely contain his excitement. "There's a lot of branches," he squeaks. "These plants are getting nice and bushy really fast thanks to the hydroponics. It took me about an hour to make this hydroponics kit so if you want me to make you one, let me know."

Given the current economic conditions, the Government is keen to encourage all sort of new entrepreneurial initiatives. But the growth of Britain's private cannabis farms is probably not quite what the Treasury had in mind.

A clandestine industry has sprung up in the bedrooms, living rooms, cellars and roof tops of Britain. Some do it for personal consumption, others grow cannabis for organised criminal networks who make millions out of what they know is a comparatively low risk, high-profit crime.

The expansion of Britain's pot farms has been so rapid that police say they are now raiding as many as 20 illegal plantations a day. More than 7,000 farms were detected last year compared with 3,000 three years earlier. Figures released yesterday by Newcastle University suggest as much as £200m worth of electricity is stolen every year by growers – many of whom are siphoning power directly from the mains supply to avoid suspiciously high electricity bills. In the past week alone raids have reported in Nottingham, Sunderland and Kensington in London.

For people like Jim, a 22-year-old mechanic from Yorkshire, the lure of easy money is simply too good an opportunity to pass up. He has been growing cannabis for the past five years, mainly at a friend's house. After learning how to grow plants properly he soon realised he had green fingers which could turn the sticky, sweet smelling buds into hard cash. "I usually just get rid of big amounts, like pounds, at once because it is too risky going little," he told The Independent. "I make around £10,000 every four months."

The explosion in cannabis farms has often been portrayed by as a relatively harmless cottage industry that allows pot smokers to steer clear of criminal networks. But the police have little time for such arguments. They say many of the small plantations are linked to criminal organisations involved in harder drugs, prostitution and trafficking.

British gangs have increasingly moved into the business, which used to be dominated by South-East Asian gangs. Without the problem of having to elude border controls or high costs of transport, the crops have become attractive to domestic criminals.

A national project has found that of 7,000 identified groups of organised criminals, a fifth are involved in cannabis cultivation or trafficking, said Commander Allan Gibson, of the Metropolitan Police. Robberies, burglaries and violence with guns have all increased because of the competition between rival groups.

The Association of Chief Police Officers released a report this week analysing the cultivation of cannabis in Britain. Officers found that gangs now try to avoid detection by splitting up large growing factories into a number of smaller plants based in homes. A number of "gardeners" are employed to manage smaller-scale operations across a number of sites.

These gardeners typically include criminals forced to work in factories to pay off "debts" to gangs. Others look to hide the factories in industrial areas so the heavy use of electricity is less likely to be spotted. Daniel Lloyd, a businessman who runs an industrial park in Telford, Shropshire, said police recently raided a unit and discovered a secret farm.

"We had no idea it was there," he said. "The man told us he needed a place to store gardening equipment and we leased him a unit. When the police raided it we discovered that he had laid out an entire farm with foil sheeting, heat lamps and insulation. Apparently he was shopped by a disgruntled friend."

For amateur horticulturalists, the internet has made it easier to grow pot successfully. There are thousands of easily accessible video tutorials and blogs detailing how to successfully nurture plants without getting caught. In one popular series, a man describes how to set up a farm in his basement from start to finish.

The success of online seed shops has also made the process easier. Although the cultivation of cannabis is illegal, there is nothing to stop someone buying, selling or importing seeds. Previously growers needed to buy them directly from dealers; now a few clicks will lead to a packet of seeds branded "Dutch passion", "purple wreck" and "cotton candy" arriving through the letterbox in a day.

Jim, the part-time grower from Yorkshire, has decided to get out of the trade now that he has landed himself a new job. "This lot is my last," he said. "I got tipped off by my friend who's a cop, saying they have been watching the house I'm growing at. I'm not starting up again because I'm now making good money working so it's not worth it."

There will be plenty of others happy to take his place.

Additional reporting by Jane Ryan and Charlie Cooper

Drug 'killed two friends on same day'

A mourner at a wake is believed to have died after taking the same drug that had killed her friend just hours earlier, police said yesterday.

Lynette Nock, 28, an accountant with a young son, joined friends at a memorial party on Saturday to honour Carl Fearon, who had been found dead that afternoon after apparently taking the dance club drug GBL the night before.

Hours later, three of the mourners collapsed and Ms Nock, described by her family as bright and kind, died shortly after being taken to hospital. Two other partygoers were discharged after treatment.

Yesterday West Midlands Police confirmed that they were investigating whether the deaths were caused by Gamma-Butyrolactone (GBL), a legal industrial cleaner that was banned for consumption as a Class C drug in 2009 after a spate of deaths.

Mr Fearon, 24, an engineer at car maker Aston Martin, was found dead at a flat in Birmingham on Saturday afternoon. His friends gathered in his memory but at 9pm West Midlands Ambulance Service were called to a house in Northfield to find Ms Nock and two men unconscious.

As they awaited the results of toxicology tests to determine whether the drug was in her system, her family said they feared her drink may have been spiked.

Barry Eveleigh, from the Birmingham Drug and Alcohol Action Team, said: "Taking GBL or GHB puts users at significant risk of unconsciousness, coma or even death. They have also been linked to drug-assisted sexual assault."

Terri Judd

Case studies: 'The money started rolling in'

"I now have a 26-plant room that puts out £10k every eight to nine weeks"

Anonymous, 30s, Doncaster

I own three cannabis "farms" in my village near Doncaster. My first plant was grown in Australia when I was 17 years old when visiting my dad. It started out as personal use but my new girlfriend had a few bills to pay and we were growing it in our house, so the choice was made and the money started rolling in. All the money was used to pay debt at first, when that cleared it was an Xbox for my step-son, a few new clothes. It was never a huge amount of money, my crops were never that huge. That was to come much later. After being broke, homeless and attempting suicide after splitting with my girlfriend, I got myself a three-bed house. [I now have] a 26-plant room that puts out between £8-10k every eight to nine weeks. [Me and a mate] go 50/50 on that. I've set up another mate with 16 plants in his attic which he has dropped to 12 due to space. Also hoping to set up an ex-squaddie next month. There are a lot of new faces in the grow shop when I go there. It's a fast-growing business.

"Grow your own and you will always know what you are smoking"

Anonymous, 16, Glasgow

I have four plants, which I grow for personal use and also to sell on. The usual yield of a cannabis flower is between 20 and 30 grams. You pay £10 for about one gram in Glasgow. If you buy more in one go you can get under-cut – dealers will say 30 grams but you'll get it back home and find you're missing eight grams. I've been smoking and selling for three or four years – making a bit of money and selling at the same time. I grow it in a four-foot tent in my cupboard. I bought the tent from a horticultural shop for £100. You need a pump, a carbonator and some UV lights – you need to get the maximum possible strength to get maximum potency from your plants. You might also need some good insecticide to keep the mites off. There's a lot of horticulture involved, you need to have green fingers. It's not about the money. I smoke more than I sell. If it was legal to grow it yourself things would be safer. You can't always trust the stuff you buy on the street, but grow your own and you will always know what you are smoking.

In numbers...

7,865 Cannabis farms were discovered in 2011/12 compared with 3,032 in 2007/8.

936 Farms found in West Yorkshire, the area of the country with the highest production rates – the equivalent of 42 factories per 100,000 people.

10 million The number of adults aged 16-59 who have used cannabis, making it the most widely consumed narcotic.

16,464 The number of cannabis production offences last year compared with 14,982 in 2010/11.