The British Government's chief drug adviser has sparked controversy by claiming ecstasy, LSD and cannabis are less dangerous than cigarettes and alcohol.

Professor David Nutt, chairman of the Advisory Council on the Misuse of Drugs, attacked the decision to make cannabis a class B drug.

He accused former home secretary Jacqui Smith, who reclassified the drug, of "distorting and devaluing" scientific research.

Prof Nutt said smoking cannabis created only a "relatively small risk" of psychotic illness. And he claimed advocates of moving ecstasy into class B from class A had "won the intellectual argument".

All drugs, including alcohol and tobacco, should be ranked by a "harm" index, he said, with alcohol coming fifth behind cocaine, heroin, barbiturates, and methadone.

Tobacco should rank ninth, ahead of cannabis, LSD and ecstasy.

Prof Nutt said: "No one is suggesting that drugs are not harmful. The critical question is one of scale and degree. We need a full and open discussion of the evidence and a mature debate about what the drug laws are for - and whether they are doing their job."

In a lecture and briefing paper for the Centre for Crime and Justice Studies at King's College, London, Prof Nutt attacked what he called the "artificial" separation of alcohol and tobacco from other, illegal, drugs.

He also repeated his claim that the risks of taking ecstasy are no worse than riding a horse.

A Home Office spokesman said: "Prof Nutt's views are his own and do not reflect the views of Government. The Government is clear - we are determined to crack down on all illegal substances and minimise their harm to health and society as a whole.

Belfast Telegraph