The UK’s former chief drugs adviser has accused Gordon Brown of reclassifying cannabis for political reasons.

Prof David Nutt also forecast that there will be more resignations from the Advisory Council on the Misuse of Drugs after he was persuaded to resign as its head.

The professor said Mr Brown had already made up his mind to reclassify cannabis as a Class B drug, despite scientific evidence.

The home secretary, Alan Johnson said that he had lost confidence in Prof Nutt, and accused him of lobbying for a change of policy.

Earlier this week, said Prof Nutt in a lecture at King’s College, London,that smoking cannabis carries a relatively small risk of psychotic illness and that it is actually less harmful than cigatettes or alcohol, but on Friday he was forced to resign after by the Home Secretary Alan Johnson who said his comments had undermined the scientific independence of the council.

Prof Nutt said to the BBC that the government has ignored advice on cannabis  on the whim of the prime minister.

Professor Nutt said:

“Until Gordon Brown took office there has never been a recommendation about drug classification from the council that has been rejected by government,” 

“Gordon Brown comes into office and soon after that he starts saying absurd things like cannabis is lethal… it has to be a Class B drug. He has made his mind up.

“We went back, we looked at the evidence, we said, ‘No, no, there is no extra evidence of harm, it’s still a Class C drug.’

“He said, ‘Tough, it’s going to be Class B.’”

Prof Nutt said drug laws should not be influenced “petty party politics” and made a comparison with interest rates, which are set by the Bank of England not the government, and said that drug classification should be done by an independent committee.

“There’s no point in having drug laws that are meaningless and arbitrary just because politicians find it useful and expedient occasionally to come down hard on drugs.

“That’s undermining the whole purpose of the drugs laws.”

Prof Nutt said his council was “very, very upset by the attitude the prime minister took over cannabis” and one of his colleagues had already told him he would resign.

 He added:

“I wouldn’t be surprised if some of the them step down – maybe all of them will”

Former government chief scientific adviser Sir David King said:

“We are witnessing a collision between science and politics – there may be significant fall-out

“I think that the government has lacked courage in backing David’s committee’s advice.  I think it’s a lack of courage from government that is a big issue here.”

Danny Shaw, from the BBC said Prof Nutt had accused the government of using the classification system as a tool to send out an anti-drugs message, instead of rating drugs by actual risk.

Liberal Democrat Shadow Home Secretary, Chris Huhne said:

“The Advisory Council on the Misuse of Drugs is simply not independent if its chair can be sacked for telling the Government what it does not want to hear.”

Liberal Democrat MP for Harrogate, Phil Willis and Chairman of the Commons science and technology committee, said: “It is disturbing if an independent scientist should be removed for reporting sound scientific advice.”

Liberal Democrat Shadow Science Minister, Evan Harris, added: “All the gains we thought we had made from the BSE enquiry have been thrown away by this act of political spite and Neanderthal wrong-headedness.

“This is the behaviour of a Minister whose judgement is impaired by the need to feed the tabloid addiction to a hard-line drugs policy.”

Harry Shapiro, director of communications at DrugScope, said:

 ”It would be naive not to expect those tasked with giving advice on drug policy to generate some controversy and debate.

“The home secretary’s decision to force the resignation of the chair of an independent advisory body is an extremely serious and concerning development and raises serious questions about the means by which drug policy is informed and kept under review.”

Phil Willis MP, Chairman of the committee, said:

“It is disturbing if an independent scientist should be removed for reporting sound scientific advice.”