Alcohol and tobacco are more harmful than many illegal drugs, including LSD, ecstasy and cannabis, according to a paper from a drugs expert.

Professor David Nutt, chairman of the government's advisory committee on the misuse of drugs, criticised politicians for "distorting" and "devaluing" the research evidence in the debate over illicit drugs.

But he also said some "top" scientific journals had published "horrific examples" of poor quality research on the alleged harm caused by some illicit drugs.

The Imperial College professor argued for a new way of classifying the harm caused by both legal and illegal drugs.

"Alcohol ranks as the fifth most harmful drug after heroin, cocaine, barbiturates and methadone. Tobacco is ranked ninth.

"Cannabis, LSD and ecstasy, while harmful, are ranked lower at 11, 14 and 18 respectively," said Nutt in the paper from the centre for crime and justice studies at King's College, London published tomorrow .

Nutt clashed with Jacqui Smith when she was home secretary after he compared the 100 deaths a year from horseriding with the 30 deaths a year linked to ecstasy. Smith also ignored the recommendation of Nutt's advisory committee that cannabis should not be reclassified from class C back to class B, leading to heavier penalties.

He criticised Smith's use of the "precautionary principle" to justify her decision to reclassify cannabis and said that by erring on the side of caution politicians "distort" and "devalue" the research evidence.

"This leads us to a position where people really don't know what the evidence is," he said adding that the initial decision to downgrade the classification of cannabis led to a fall in the use of the drug.

Nutt acknowledged there was a "relatively small risk" of psychotic illness linked to cannabis use. But he argued that to prevent one episode of schizophrenia it would be necessary to "stop 5,000 men aged 20 to 25 from ever using" cannabis.

Nutt also renewed his support for reclassifying ecstasy from a class A drug to class B, saying the advisory committee "won the intellectual argument" over the issue but obviously didn't win the decision after the home secretary vetoed the move.

He said the quality of some research papers about cannabis and ecstasy was so poor the articles had to be retracted. Richard Garside, director of the centre for crime and justice, said Nutt's briefing paper gave an insight into what drugs policy might look like if it was based on the research evidence rather than political or moral positioning.