The government's former chief drug adviser today accused the prime minister, Gordon Brown, of tightening the law on cannabis for political reasons.

Professor David Nutt warned that other experts on the Advisory Council on the Misuse of Drugs (ACMD) could resign in protest at his sacking by the home secretary, Alan Johnson, yesterday.

Nutt was forced to quit after he accused ministers of "devaluing and distorting" the scientific evidence over illicit drugs when they decided last year to reclassify cannabis from class C to class B against the advice of the ACMD.

Nutt told the BBC today that Brown had "made up his mind" to reclassify cannabis despite evidence to the contrary.

"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'."

Speaking on BBC Radio 4's Today programme, Nutt said: "He is the first prime minister, this is the first government, that has ever in the history of the Misuse of Drugs Act gone against the advice of its scientific panel.

"And then it did it again with ecstasy and I have to say it's not about [me] overstepping the line, it's about the government overstepping the line. They are making scientific decisions before they've even consulted with their experts.

"I know that my committee was very, very upset by the attitude the prime minister took over cannabis. We actually formally wrote to him to complain about it," he said. "I wouldn't be surprised if some of them stepped down. Maybe all of them will."

Nutt's sacking is likely to raise concerns among scientists over the independence of advice to the government and may trigger further resignations. The Home Office describes the ACMD as an independent expert body that advises on drug-related issues, including recommendations on classification under the 1971 Misuse of Drugs Act.

It is not thought that the home secretary spoke directly to Nutt before requesting his resignation in writing.

Johnson accused the professor of going beyond his remit as an evidence-based scientist and accused him of "lobbying for a change in government policy" rather than giving impartial advice.

"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," Johnson wrote to Nutt.

"As my lead adviser on drugs harm I am afraid the manner in which you have acted runs contrary to your responsibilities.

"I cannot have public confusion between scientific advice and policy and have therefore lost confidence in your ability to advise me as chair of the ACMD."

The decision followed the publication of a paper by the Centre for Crime and Justice at King's College London, based on a lecture Nutt delivered in July. He repeated his familiar view that illicit drugs should be classified according to the actual evidence of the harm they cause and pointed out that alcohol and tobacco caused more harm than LSD, ecstasy and cannabis.

He accused the former home secretary, Jacqui Smith, of distorting and devaluing scientific research when she reclassified cannabis, and repeated his claim that the risks of taking ecstasy were no worse than riding a horse.

The charity DrugScope's director of communications, Harry Shapiro, said: "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."

Richard Garside, the director of the Centre for Crime and Justice at King's College London, accused Johnson of undermining scientific research.

He said: "I'm shocked and dismayed that the home secretary appears to believe that political calculation trumps honest and informed scientific opinion."

Sir Leszek Borysiewicz, chief executive of the Medical Research Council (MRC), said: "It is crucial that UK policy is based on evidence and that scientists are able to offer unfettered advice without the fear of reprisal. This principle should be the backbone of scientific engagement with government."

Neuroscientist Professor Colin Blakemore, former chief executive of the MRC, said he believed Prof Nutt was trying to "inform debate".

"If ministers decide to go against the recommendations of their own experts, I really think the public is entitled to know why," he told Sky News.