They heard, too, from specialist doctors, such as Alexander Lambert of New York's Bellevue hospital, who explained that "the herd instinct is obliterated by heroin, and the herd instincts are the ones which control the moral sense ... Heroin makes much quicker the muscular reaction and therefore is used by criminals to inflate them, because they are not only more daring, but their muscular reflexes are quicker." Senior police, a prison governor and health officials all added their voices. Dr S Dana Hubbard, of the New York City health department, captured the heart of the evidence: "Heroin addicts spring from sin and crime ... Society in general must protect itself from the influence of evil, and there is no greater peril than heroin."

The congressmen had heard much of this before and now they acted decisively. They resolved to stop the manufacture and use of heroin for any purpose in the United States and to launch a worldwide campaign of prohibition to try to prevent its manufacture or use anywhere in the world. Within two months, their proposal had been passed into law with the unanimous backing of both houses of the US Congress. The war against drugs was born.

To understand this war and to understand the problems of heroin in particular, you need to grasp one core fact. In the words of Professor Arnold Trebach, the veteran specialist in the study of illicit drugs: "Virtually every 'fact' testified to under oath by the medical and criminological experts in 1924 ... was unsupported by any sound evidence." Indeed, nearly all of it is now directly and entirely contradicted by plentiful research from all over the world. The first casualty of this war was truth and yet, 77 years later, the war continues, more vigorous than ever, arguably the longest-running conflict on earth.

Drugs and fear go hand in hand. The war against drugs is frightening - but not, in reality, for the reasons which are claimed by its generals. The untold truth about this war, which has now sucked in every country in the developed world, is that it creates the very problem which it claims to solve. The entire strategy is a hoax, with the same effect as an air force which bombs its own cities instead of its enemy's. You have to go back to the trenches of Flanders to find generals who have been so incompetent, so dishonest, so awesomely destructive towards those for whom they claim to care.

The core point is that the death and sickness and moral collapse which are associated with class A drugs are, in truth, generally the result not of the drugs themselves but of the black market on which they are sold as a result of our strategy of prohibition. In comparison, the drugs themselves are safe, and we could turn around the epidemic of illness and death and crime if only we legalised them. However, it is a contemporary heresy to say this, and so the overwhelming evidence of this war's self-destructive futility is exiled from almost all public debate now, just as it was when those congressmen met.

Take heroin as a single example. And it's a tough example. In medical terms, it is simply an opiate, technically known as diamorphine, which metabolises into morphine once it enters its user's body. But, in terms of the war against drugs, it is the most frightening of all enemies. Remember all that those congressmen were told about "the great peril". Remember the Thatcher government's multimillion pound campaign under the slogan "Heroin screws you up". Think of Tony Blair at the 1999 Labour party conference fulminating about the "drug menace" or of William Hague last year calling for "a stronger, firmer, harder attack on drugs than we have ever seen before". And now look at the evidence.

Start with the allegation that heroin damages the minds and bodies of those who use it, and consider the biggest study of opiate use ever conducted, on 861 patients at Philadelphia General hospital in the 20s. It concluded that they suffered no physical harm of any kind. Their weight, skin condition and dental health were all unaffected. "There is no evidence of change in the circulatory, hepatic, renal or endocrine functions. When it is considered that some of these subjects had been addicted for at least five years, some of them for as long as 20 years, these negative observations are highly significant."

Check with Martindale, the standard medical reference book, which records that heroin is used for the control of severe pain in children and adults, including the frail, the elderly and women in labour. It is even injected into premature babies who are recovering from operations. Martindale records no sign of these patients being damaged or morally degraded or becoming criminally deviant or simply insane. It records instead that, so far as harm is concerned, there can be problems with nausea and constipation.

Or go back to the history of "therapeutic addicts" who became addicted to morphine after operations and who were given a clean supply for as long as their addiction lasted. Enid Bagnold, for example, who wrote the delightful children's novel, National Velvet, was what our politicians now would call "a junkie", who was prescribed morphine after a hip operation and then spent 12 years injecting up to 350mg a day. Enid never - as far as history records - mugged a single person or lost her "herd instinct", but died quietly in bed at the age of 91. Opiate addiction was once so common among soldiers in Europe and the United States who had undergone battlefield surgery that it was known as "the soldiers' disease". They spent years on a legal supply of the drug - and it did them no damage.

We cannot find any medical research from any source which will support the international governmental contention that heroin harms the body or mind of its users. Nor can we find any trace of our government or the American government or any other ever presenting or referring to any credible version of any such research. On the contrary, all of the available research agrees that, so far as harm is concerned, heroin is likely to cause some nausea and possibly severe constipation and that is all. In the words of a 1965 New York study by Dr Richard Brotman: "Medical knowledge has long since laid to rest the myth that opiates observably harm the body." Peanut butter, cream and sugar, for example, are all far more likely to damage the health of their users.

Now, move on to the allegation that heroin kills its users. The evidence is clear: you can fatally overdose on heroin. But the evidence is equally clear, that - contrary to the claims of politicians - it is not particularly easy to do so. Opiates tend to suppress breathing, and doctors who prescribe them for pain relief take advantage of this to help patients with lung problems. But the surprising truth is that, in order to use opiates to suppress breathing to the point of death, you have to exceed the normal dose to an extreme degree. Heroin is unusually safe, because - contrary to what those US congressmen were told in 1924 - the gap between a therapeutic dose and a fatal dose is unusually wide.

Listen, for example, to Dr Teresa Tate, who has prescribed heroin and morphine for 25 years, first as a cancer doctor and now as medical adviser to Marie Curie Cancer Care. We asked her to compare heroin with paracetamol, legally available without prescription. She told us: "I think that most doctors would tell you that paracetamol is actually quite a dangerous drug when used in overdose; it has a fixed upper limit for its total dose in 24 hours and if you exceed that, perhaps doubling it, you can certainly put yourself at great risk of liver failure and of death, whereas with diamorphine, should you double the dose that you normally were taking, I think the consequence would be to be sleepy for a while and quite possibly not much more than that and certainly no permanent damage as a result." Contrary to the loudly expressed view of so many politicians, this specialist of 25 years' experience told us that when heroin is properly used by doctors, it is "a very safe drug".

Until the American prohibitionists closed him down in the 20s, Dr Willis Butler ran a famous clinic in Shreveport, Louisiana, for old soldiers and others who had become addicted to morphine after operations. Among his patients, he included four doctors, two church ministers, two retired judges, an attorney, an architect, a newspaper editor, a musician from the symphony orchestra, a printer, two glass blowers and the mother of the commissioner of police. None of them showed any ill effect from the years which they spent on Dr Butler's morphine. None of them died as a result of his prescriptions. And, as Dr Butler later recalled: "I never found one we could give an overdose to, even if we had wanted to. I saw one man take 12 grains intravenously at one time. He stood up and said: 'There, that's just fine,' and went on about his business."

Heroin can be highly addictive - which is a very good reason not to start taking it. In extreme doses, it can kill. But the truth which has been trampled under the cavalry of the drug warriors is that, properly prescribed, pure heroin is a benign drug. The late Professor Norman Zinberg, who for years led the study of drug addiction at Harvard Medical School, saw the lies beneath the rhetoric: "To buttress our current programme, official agencies, led originally by the old Federal Bureau of Narcotics, have constructed myth after myth. When pushers in schoolyards, 'drug progression', drugs turning brains to jelly, and other tales of horror are not supported by facts, they postulate and publicise others: 'drugs affect chromosomes'; 'drugs are a contagious disease'. Officials go on manufacturing myths such as the chromosome scare long after they are disproved on the self-righteous assumption that if they have scared one kid off using drugs, it was worth the lie."

Take away the lies and the real danger becomes clear - not the drugs, but the black market which has been created directly by the policy of prohibition. If ever there is a war crimes trial to punish the generals who have gloried in this slaughter of the innocent, the culprits should be made to carve out in stone: "There is no drug known to man which becomes safer when its production and distribution are handed over to criminals."

Heroin, so benign in the hands of doctors, becomes highly dangerous when it is cut by black-market dealers - with paracetamol, drain cleaner, sand, sugar, starch, powdered milk, talcum powder, coffee, brick dust, cement dust, gravy powder, face powder or curry powder. None of these adulterants was ever intended to be injected into human veins. Some of them, such as drain cleaner, are simply toxic and poison their users. Others - sand or brick dust - are carried into tiny capillaries and digital blood vessels where they form clots, cutting off the supply of blood to fingers or toes. Very rapidly, venous gangrene sets in, the tissue starts to die, the fingers or toes go black and then have only one destiny: amputation. Needless suffering - inflicted not by heroin, but by its black-market adulterants.

Street buyers cannot afford to waste any heroin - and for that reason, they start to inject it, because smoking or snorting it is inefficient. The Oxford Handbook of Clinical Medicine records that a large proportion of the illness experienced by black-market heroin addicts is caused by wound infection, septicaemia, and infective endocarditis, all due to unhygienic injection technique. Street users invariably suffer abscesses, some of them of quite terrifying size, from injecting with infected needles or drugs. Those who inject repeatedly into the same veins or arteries will suffer aneurisms - the walls of the artery will weaken and bulge; sometimes they will start to leak blood under the skin; sometimes, these weakened arteries will become infected by a dirty needle and rupture the skin, leaving the user to bleed to death.

In the mid 90s, the World Health Organisation estimated that 40% of recent Aids cases internationally had been caused by drug users sharing injecting equipment. The British record on Aids is better because in the late 80s the government quietly broke with its prohibition philosophy and started to provide clean needles. Nevertheless, by June last year, 1,000 black-market drug users in this country had died of Aids which was believed to have been contracted from dirty needles. More needless misery and death.

Far worse, however, is the spread of hepatitis C, which can kill by causing cirrhosis and sometimes cancer in the liver. The official estimate is that 300,000 people in this country are now infected. Dr Tom Waller, who chairs Action on Hepatitis C, says the truth is likely to be much worse. And almost all of these victims are black-market drug users who contracted the disease by sharing dirty injecting equipment. Dr Waller says there is now a "major epidemic", threatening the lives of "a great many people". Needlessly.

Street buyers buy blind and so they will overdose accidentally: they have no way of telling how much heroin there is in their deal. Dr Russell Newcombe, senior lecturer in addiction studies at John Moores University in Liverpool, has found the purity of street heroin varying from 20% to 90%. "Users can accidentally take three or four times as much as they are planning to," he says. It is peculiarly ironic that governments set out to protect their people from a drug which they claim is dangerous by denying them any of the safeguards and information which they insist must apply to the consumption of drugs which they know to be harmless. (Compare, for example, the mandatory information on the side of a bottle of vitamin C tablets with the information available to a black-market heroin user.)

Street buyers often run short of supplies - and so they mix their drug with anything else they can get their hands on, particularly alcohol. Heroin may be benign, but if you mix it with a bottle of vodka or a handful of sedatives, your breathing is likely to become extremely depressed. Or it may just stop. In any event, whether it is poisonous adulterants or injected infection; whether it is death by accidental overdose or death by polydrug use: it is the black market which lies at the root of the danger. The healthiest route, of course, is not to take the drug at all: but for those who are addicted, prohibition inflicts danger and death. Needlessly. Water would become dangerous if it were banned and handed over to a criminal black market.

The same logic applies to drugs which, unlike heroin, are inherently harmful - such as alcohol, which is implicated in organic damage (liver) and social problems (violence, dangerous driving). American bootleggers brewed their moonshine with adulterants such as methylated spirits, which can cause blindness. (Hence the proliferation of blind blues singers.) And there are documented cases of drinkers during prohibition injecting alcohol, with all of the attendant dangers. (It is instructive to look back on the prohibitionists' efforts to justify their war against alcohol with hugely inflated statements of its danger. In his book on the history of drugs, Emperors of Dreams, Mike Jay records the claims that alcohol was an "environmental poison" which generated cretinism and several otherwise unrecognised syndromes including "blastophoric degeneration" and "alcoholic diathesis".)

The risks of consuming LSD and ecstasy are increased enormously by their illegal and unsupervised manufacture. Nobody knows what they are swallowing. Yet, when a Brighton company developed a test to check the purity of ecstasy, the government's drugs adviser, Keith Hellawell (whose contract has just been suspended), condemned it and warned that the company risked prosecution. It is the same with black-market amphetamines: speed alone may not kill, but speed with a blindfold is highly likely to finish you off.

In the same way, the classic signs of social exclusion among addicts are the product not of their drug but of the illegality of the drug. If addicts fail to work, it is not because heroin has made them work-shy, but because they spend every waking minute of the day hustling. If addicts break the law, it is not because the drug has corrupted their morality, but because they are forced to steal to pay black-market prices. If addicts are thin, it is not because the drug has stripped away their flesh, but because they spend every last cent on their habit and have nothing left for food. Over and over again, it is the black market, which has been created by the politicians, which does the damage.

Keith Hellawell, the man to whom the government turned for advice on drugs, appeared to know none of this. When we interviewed him for a television programme, he insisted that heroin itself was dangerous and then repeatedly dodged requests to come up with any evidence at all to justify his claim. Subsequently, when we offered his department as much time as it would like to find any evidence, it failed to come up with anything at all and passed the question to the Department of Health, which also failed. It is fair to conclude that the government's former drugs adviser did not know the first thing about heroin.

The confusion between the effect of the drug and the effect of the black market is exacerbated not only because of government policy but also because government statistics completely ignore this distinction, with the result that teams of researchers study drug policy, use compromised statistics and simply recycle the confusion, thus providing politicians with yet more false fuel for their fire. Home Office figures on drug deaths, for example, are hopelessly compromised. Eighteen months ago, the Department of Health, which might have been expected to know better, produced new guidelines for doctors dealing with drug users and recorded the following: "Generally there is a greater prevalence of certain illnesses among the drug misusing population, including viral hepatitis, bacterial endocarditis, HIV, tuberculosis, septicaemia, pneumonia, deep vein thrombosis, pulmonary emboli, abscesses and dental disease." All of it true of the black market. None of it true of the drug. No attempt to make the distinction.

The black market damages not only drug users but the whole community. Britain looks back at the American prohibition of alcohol in the 20s and shudders at the stupidity of a policy which generated such a catastrophic crimewave. Yet in this country, now, the prohibition of drugs has generated a crime boom of staggering proportions. Research suggests that in England and Wales, a hard core of black-market users is responsible for some £1.5bn worth of burglary, theft and shoplifting each year - they are stealing £3.5m worth of property a day. As a single example, Brighton police told us they estimate that 75% of their property crime is committed by black-market drug users trying to fund their habit. And yet governments refuse to be tough on the cause of this crime: their own prohibition policy.

The global version of this damage was put succinctly by Senator Gomez Hurtado, former Colombian ambassador to France and a high court judge, who told a 1993 conference: "Forget about drug deaths and acquisitive crime, about addiction and Aids. All this pales into insignificance before the prospect facing the liberal societies of the west, like a rabbit in the headlights of an oncoming car. The income of the drug barons is an annual $500,000m, greater than the American defence budget. With this financial muscle they can suborn all the institutions of the state and, if the state resists, with this fortune they can purchase the firepower to outgun it. We are threatened with a return to the dark ages of rule by the gang. If the west relishes the yoke of the tyrant and the bully, current drug policies promote that end."

Having attacked and maimed and killed the very people they claimed to be protecting; having inflicted a crime wave on the same communities which they said they were defending; having run up a bill which now costs us some £1.7bn a year in this country alone: this war's generals might yet have some claim to respect if they were able to show that they had succeeded in their original objective of stopping or, at least, of cutting the supply of prohibited drugs. They cannot.

In December 1999, the chief constable of Cleveland police, Barry Shaw, produced a progress report on the 1971 Misuse of Drugs Act, which marked the final arrival of US drugs prohibition in this country: "There is overwhelming evidence to show that the prohibition-based policy in this country since 1971 has not been effective in controlling the availability or use of proscribed drugs. If there is indeed a war against drugs, it is not being won ... Illegal drugs are freely available, their price is dropping and their use is growing. It seems fair to say that violation of the law is endemic, and the problem seems to be getting worse despite our best efforts."

Mr Shaw was able to point to a cascade of evidence to support his view: between 1987 and 1997, there had been a tenfold increase in the seizure of illicit drugs, and yet the supply on the streets was so strong that the price of these drugs had kept dropping; in 1970, only 15% of people had used an illegal drug, but by 1995, 45% had; in 1970, 9,000 people were convicted of a drugs offence but in 1995 94,000 were. The Home Office responded to the chief constable's report with complete silence: they refused even to acknowledge receiving it. Internal reports from the American Drugs Enforcement Agency confirm the chief constable's conclusion. (They say Britain now produces so much cannabis that we actually export it to Holland.)

Prohibition has not merely failed to cut the supply of illicit drugs: it has actively spread drug use. The easiest way for new users to fund their habit is to sell drugs and consume the profit; so they go out and find new users to sell to; so it is that when one child in the classroom starts using, others soon join in; one user in the street and neighbours soon follow. Black-market drug use spreads geometrically. The Health Education Authority in 1995 found that 70% of people aged between 11 and 35 had been offered drugs at some time. Pushers push. When Britain began to impose prohibition of heroin, in 1968, there were fewer then 500 heroin addicts in Britain - a few jazz musicians, some poets, some Soho Chinese. Now, the Home Office says there may be as many as 500,000. This is pyramid selling at its most brilliantly effective.

In private, the Home Office's best defence is that it is so short of reliable intelligence on drugs that nobody can finally prove that the war is lost: we simply don't know how much heroin or cocaine is imported, or how many people are using it.

Keith Hellawell argued that the 30 years since the Misuse of Drugs Act do not really count, because, until he took over, British governments did not have a real strategy. He told us he was supporting new international tactics (which he could not divulge) and was now seeing figures (which he could not give us) to suggest finally they were going to succeed. This recalls earlier declarations that "We have turned the corner on drug addiction" (President Nixon, 1973), or "Heroin availability continues to shrink" (DEA, 1978). In the meantime, world heroin production has tripled in the past decade, cocaine production has doubled and, in the foreign secretary's Blackburn constituency, police say drug use in the Asian community has soared by 300% in four years.

But the underlying point is even more worrying: once you understand that the real danger comes from the black market and not from the drug, you can see that even if, with some magic formula, the generals started to cut the supply of these drugs, the result would be disastrous. The price of heroin, for example, would start to rise and, since there is no evidence at all that heroin addicts cut their consumption to fit their wallets, they would have to commit more crime to fund their habits. And if the dealers also responded like good entrepreneurs, they would try to keep their prices down by adding even more pollutants to the heroin, thus increasing the health risks to users.

This government has not begun to consider legalisation. No matter the truth about the danger and the death, no matter the truth about the cause of crime, the position is, as Jack Straw put it to the 1997 Labour conference: "We will not decriminalise, legalise or legitimise the use of drugs." Why? The obvious answer was offered to us by Paul Flynn, Labour backbencher and staunch opponent of prohibition: "It is being fuelled by politicians who are vote gluttons, who believe that there is popularity and votes to be gained by appearing to be tough on drugs."

While Keith Hellawell and other prohibitionists are embarrassed by their screaming lack of success, those who want to legalise can point to clear evidence that providing a clean supply of drugs will help with the physical and mental health of users, will cut crime in the community and drain the life out of the black market.

The Swiss, for example, in 1997 reported on a three-year experiment in which they had prescribed heroin to 1,146 addicts in 18 locations. They found: "Individual health and social circumstances improved drastically ... The improvements in physical health which occurred during treatment with heroin proved to be stable over the course of one and a half years and in some cases continued to increase (in physical terms, this relates especially to general and nutritional status and injection- related skin diseases) ... In the psychiatric area, depressive states in particular continued to regress, as well as anxiety states and delusional disorders ... The mortality of untreated patients is markedly higher." They also reported dramatic improvements in the social stability of the addicts, including a steep fall in crime.

There are equally impressive results from similar projects in Holland and Luxembourg and Naples and, also, in Britain. In Liverpool, during the early 1990s, Dr John Marks used a special Home Office licence to prescribe heroin to addicts. Police reported a 96% reduction in acquisitive crime among a group of addict patients. Deaths from locally acquired HIV infection and drug-related overdoses fell to zero. But, under intense pressure from the government, the project was closed down. In its 10 years' work, not one of its patients had died. In the first two years after it was closed, 41 died.

There is room for debate about detail. Should we supply legalised drugs through GPs or specialist clinics or pharmacists? Should we continue to supply opiate substitutes, such as methadone, as well as heroin? Should the supply be entirely free of charge to guarantee the extinction of the black market? How would we use the hundreds of millions of pounds which would be released by the "peace dividend"? But, if we have any compassion for our drug users, if we have any intention of tackling the causes of crime, if we have any honesty left in our body politic, there is no longer any room for debate about the principle. Continue the war against drugs? Just say no.

Additional reporting by Jane Cassidy.

Nick Davies's programme, Drugs - The Phoney War, is to be shown on Channel 4 at 9pm on Tuesday June 19.