Some of Britain's leading drug experts demand today that the government's classification regime be scrapped and replaced by one that more honestly reflects the harm caused by alcohol and tobacco. They say the current ABC system is "arbitrary" and not based on evidence.

The scientists, including members of the government's top advisory committee on drug classification, have produced a rigorous assessment of the social and individual harm caused by 20 substances, and believe this should form the basis of any future ranking.

By their analysis, alcohol and tobacco are rated as more dangerous than cannabis, LSD and ecstasy.

They say that if the current ABC system is retained, alcohol would be rated a class A drug and tobacco class B.

"We face a huge problem," said Colin Blakemore, chief executive of the Medical Research Council and an author of the report, which is published in the Lancet medical journal. "Drugs ... have never been more easily available, have never been cheaper, never been more potent and never been more widely used.

"The policies we have had for the last 40 years ... clearly have not worked in terms of reducing drug use. So I think it does deserve a fresh look. The principal objective of this study was to bring a dispassionate approach to what is a very passionate issue."

David Nutt, a psychopharmacologist at Bristol University and member of the Advisory Council on Misuse of Drugs (ACMD) which advises ministers on drug policy, added: "What we are trying to say is we should review the penalties in the light of the harms and try to have a more proportionate legal response.

"The point we are making is that all drugs are dangerous, even the ones that people know and love and use regularly like alcohol."

Professor Nutt and his team analysed the evidence of harm caused by 20 drugs including heroin, cocaine, cannabis, ecstasy, LSD and tobacco.

They asked a group of 29 consultant psychiatrists who specialise in addiction to rate the drugs in nine categories. Three of these related to physical harm, three to the likelihood of addiction and three to social harms such as healthcare costs. The team also extended the analysis to another group of 16 experts spanning several fields including chemistry, pharmacology, psychiatry, forensics, police and legal services.

The final rankings placed heroin and cocaine as the most dangerous of the 20 drugs. Alcohol was fifth, the class C drug ketamine sixth and tobacco was in ninth place, just behind amphetamine or "speed".

Cannabis was 11th, while LSD and ecstasy were 14th and 18th respectively. The rankings do take into account new evidence that specially cultivated "skunk" varieties of cannabis available now are two to three times stronger than traditional cannabis resin.

Evan Harris MP, the Liberal Democrats' science spokesman, said the paper undermines the government's claim that drug policy is evidence-based. "This comes from the top echelons of the government's own advisory committee on the misuse of drugs. It blows a hole in the government's current classification system for drugs." He said the ACMD should make recommendations to ministers on how to change drug policy based on the findings.

But the shadow home secretary, David Davis, rejected any changes that would confuse the public. "Drugs wreck lives, destroy communities and fuel other sorts of crime - especially gun and knife crime. Thanks to the government's chaotic and confused approach to drugs policy, young people increasingly think it is OK to take drugs," he said, adding that he was against downgrading of ecstasy. "It is vital nothing else leads young people to believe drugs are OK."

The position of ecstasy near the bottom of the list was defended by Prof Nutt, who said that apart from some tragic isolated cases ecstasy is relatively safe. Despite about a third of young people having tried the drug and around half a million users every weekend, it causes fewer than 10 deaths a year. One person a day is killed by acute alcohol poisoning and thousands more from chronic use.

Prof Nutt said young people already know ecstasy is relatively safe, so having it in class A makes a mockery of the entire classification system for them. "The whole harm-reduction message disappears because people say, 'They are lying.' Let's treat people as adults, tell them the truth and hopefully work with them to minimise use."

Another advantage of the new system, according to Professor Blakemore, is that it would be easy to tweak the rankings based on new evidence.

The public furore over the downgrading of cannabis from B to C, he said, showed how hard it is to change drug classifications once they are fixed. "[Our system] would be easy to use on a rolling basis, to reassess the harms of drugs as evidence developed," he said.