Prof Nutt said the PM's stance on cannabis was 'absurd'
The UK's former chief drugs adviser has accused Gordon Brown of reclassifying cannabis for political reasons.
Prof David Nutt also predicted there would be further resignations from the Advisory Council on the Misuse of Drugs after he was made to quit as its head.
The professor said Mr Brown had "made up his mind" to make cannabis a Class B drug, despite evidence to the contrary.
The home secretary said he had "lost confidence" in Prof Nutt, accusing him of "lobbying for a change in policy".
Earlier this week, Prof Nutt used a lecture at King's College, London, to say that smoking cannabis created only a "relatively small risk" of psychotic illness and it was actually less harmful than nicotine or alcohol.
But on Friday he was forced to quit after receiving a letter from Home Secretary Alan Johnson who said his comments had undermined the scientific independence of the council.
Prof Nutt told the BBC the government had ignored advice on cannabis "on the whim of the prime minister".
"Until Gordon Brown took office there has never been a recommendation about drug classification from the council that has been rejected by government," he said.
I think it's a lack of courage from government that is a big issue here
Sir David King Former government chief scientific adviser
"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 compared them to interest rates, which are set by the Bank of England not the government.
In the same way, he said, an independent committee should be set up to rule on drug classifications.
"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 had already told him he would resign.
"I wouldn't be surprised if some of the them step down - maybe all of them will," he added.
Former government chief scientific adviser Sir David King said Prof Nutt had "stepped over the line" in criticising politicians, but it was "absolutely wrong" of Mr Johnson to try to stop him making his opinions about cannabis public.
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"I think that the government has lacked courage in backing David's committee's advice," Sir David said. "I think it's a lack of courage from government that is a big issue here."
The BBC's Danny Shaw said Prof Nutt had accused the government of using the classification system as a tool to send out an anti-drugs message, rather than to rate drugs in terms of actual risk.
Shadow home secretary Chris Grayling said the sacking had been "an inevitable decision" after Prof Nutt's "latest ill-judged contribution to the debate".
But Liberal Democrat home affairs spokesman Chris Huhne said the decision to sack the adviser had been "disgraceful".
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."
Meanwhile, the Commons Science and Technology Select Committee has asked Mr Johnson to clarify why Prof Nutt was removed at a time when independent scientific advice was essential.
Phil Willis MP, Chairman of the committee, said: "It is disturbing if an independent scientist should be removed for reporting sound scientific advice."
But Prof Nutt's predecessor at the ACMD, Sir Michael Rawlins, said ministers had rejected advice in the past and would continue to do so.
He said: "Governments may well have good reasons for taking an alternative view... When that happens, then the government should explain why it's ignoring the particular advice and I think that's generally accepted as the principle."
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