The Government's chief drugs adviser has been forced to resign in the wake of the row over the dangers of class A drugs.

Home Secretary Alan Johnson asked Professor David Nutt, who heads up the Psychopharmacology Unit at Bristol University, to resign as chairman of the Advisory Council on the Misuse of Drugs (ACMD), saying he had "lost confidence" in his ability to give impartial advice.

He accused Prof Nutt of going beyond his remit as an evidence-based scientist and accused him of "lobbying for a change in Government policy".

But Prof Nutt hit back, accusing the Government of "misleading" the public in their messages about drugs and of "Luddite" tendencies.

He linked his sacking to "political" considerations, citing the forthcoming General Election.

Professor Nutt sparked controversy this week when he said ecstasy and LSD were less harmful than alcohol and cigarettes, and criticised the decision to upgrade cannabis to class B.

Speaking after he agreed to step down, he said: "It's unusual political times, I suppose, elections and all that. It's disappointing.

"But politics is politics and science is science and there's a bit of a tension between them sometimes."

He attacked Prime Minister Gordon Brown for making what he said were "completely irrational" statements about cannabis.

Confessing to be "extremely surprised" by the decision, he said: "The danger is they (politicians) are misleading us. The scientific evidence is there, it's in all the reports we published."

In his letter demanding Prof Nutt's resignation, Mr Johnson wrote: "It is important that the Government's messages on drugs are clear and, as an adviser, you do nothing to undermine the public understanding of them."