The Government’s chief drugs adviser has been sacked for “crossing the line” into political campaigning against drugs laws.

The decision was made by Home Secretary Alan Johnson after Professor David Nutt made headlines for claiming that cannabis and ecstasy were less dangerous than cigarettes and alcohol.

Mr Johnson told Prof Nutt that he had “lost confidence” in his ability to give impartial advice.

Prof Nutt’s claims, published in an academic briefing last week, were the latest in a series of criticisms of Government policy on drugs, including the reclassification of cannabis back to class B.

Prof Nutt also repeated his view that taking ecstasy is no more dangerous than riding a horse.

When he made the same claim earlier this year former Home Secretary Jacqui Smith made him apologise to the families of people killed by ecstasy.

The incident caused alarm at the Home Office, with one source saying: “Anything that appears to downgrade the dangers of drugs is just not acceptable and it should not have been said.”

Justifying his decision in a TV interview, Mr Johnson criticised Prof Nutt’s reaction to the tougher cannabis classification.

He said: “You cannot have a chief adviser at the same time stepping into the public field and campaigning against Government decisions.

He added that it was not the job of a scientific adviser to “just keep coming back and back” to overturn ministerial decisions.

Prof Nutt responded by reiterating his beliefs on the relative dangers of illegal drugs.

He told the Mail on Sunday newspaper: “I’m sure my children have tried drugs. In fact, some have told me they have.

“But I’m not as concerned about this as I am about alcohol.”

Two of Prof Nutt’s former colleagues on the Government’s Advisory Council on the Misuse of Drugs have resigned in protest at his sacking.

Colin Hart, Director of the The Christian Institute, said: “Drugs policy is a moral and political issue. Scientists cannot claim to have a monopoly on common sense.”