The class A drug Ecstasy, possession of which can result in a seven-year prison sentence, is placed near the bottom of the league table which lists "harm scores'' for different substances.

LSD, another class A drug, is also considered relatively safe despite its powerful hallucinogenic properties.

Cannabis, recently downgraded to class C, occupies a middle position. It is rated more dangerous than Ecstasy, LSD and the dance floor drug GHB, but less harmful than tobacco.

The table, published in The Lancet medical journal, was drawn up by a team of highly respected scientists led by Professor David Nutt, from the University of Bristol, and Professor Colin Blakemore, chief executive of the Medical Research Council.

It is intended to be a model for policy makers which is more scientifically based than the current Misuse of Drugs Act system that attaches "a, b, and c'' labels to illicit drugs.

The scientists identified three main factors that together determined the harmfulness of a controlled substance.

These were: the physical harm to the individual user caused by the drug, the tendency of the drug to induce dependence, and the effect of the drug's use on families, communities and society.

Each of these categories was split into three sub-components, providing nine parameters of risk.

Independent experts, including psychiatrists, chemists and forensic scientists, were invited to rate each of these parameters on a four-point scale. Zero denoted "no risk'', 1 "some risk'', 2 "moderate risk'' and 3 "extreme risk''.

Scores were combined to produce estimates of harm for each of 20 different drugs.

Unsurprisingly, the results placed heroin at the head of the table with an overall "harm score'' of 2.7, followed by cocaine which scored 2.3.

But more controversially, alcohol is ranked as the fifth most dangerous drug, scoring just under 2 on the table. Tobacco is in ninth position, with a score of 1.7. Ecstasy is listed at number 18 and scores just over 1.1.

Speaking at a news briefing in London, Prof Blakemore said: "Alcohol and tobacco are way up there in the league table, with alcohol being not very far behind demonised terrors of the street like heroin.''

He pointed out that at present there was no rational, evidence-based method for assessing the harmfulness of drugs. "We have tried to develop such a method,'' said Prof Blakemore.

"We hope that policy makers will take note of the fact that the resulting ranking of drugs differs substantially from their classification in the Misuse of Drugs Act and that alcohol and tobacco are judged more harmful than many illegal substances.''

The scientists said they were not pressing the Government to adopt the new system, but hoped it would spark a debate that might lead to future reforms.