Mass walkout threatened as axed adviser David Nutt accuses Gordon Brown of being first prime minister to go against advice of his scientific panel

The government was at the centre of a furious backlash from leading scientists last night following its sacking of Britain's top drugs adviser.

The decision by the home secretary, Alan Johnson, to call on Professor David Nutt to resign as chairman of the Advisory Council on the Misuse of Drugs (ACMD) has thrown the future of the respected independent body into severe doubt. There were claims last night that many of those who sit on the 31-strong council – which advises ministers on what evidence there is of harm caused by drugs – may resign en masse, raising serious doubts about how ministers will justify policy decisions.

Several were this weekend seeking urgent reassurances from the government that it will not try to control their agenda and will allow them to speak out before they decide whether to quit. One is said to have already resigned.

The government's decision to dismiss Nutt came after he wrote a paper for the Centre for Crime and Justice Studies (CCJS) at King's College London that questioned the "artificial" separation of alcohol and tobacco from illegal drugs.

Nutt told the Observer he had received hundreds of messages of support and had been contacted by several members of the council. "I actually think it might be an untenable position," Nutt said of the chairmanship. "I can't believe that any independent-minded scientists would want to take it on. People will think, if you can't speak your mind and be honest about what you think, why take on the job? It might be that the council becomes unviable."

He said he had not approached members of the council – who include police officers and social services professionals as well as medical experts – but about a third had already contacted him.

"All the ones that have contacted me are considering their positions," he said. "There is uniform support, uniform horror at what happened. We have been abused by government, misused by government."

Nutt accused the former home secretary, Jacqui Smith, of "distorting and devaluing" scientific research. He said Smith's decision to reclassify cannabis meant she had fallen victim to a "skunk scare", and in another dig at the government claimed that advocates of downgrading ecstasy from class A to class B had "won the intellectual argument".

Speaking on BBC Radio 4's Today programme yesterday, Nutt was also fiercely critical of Gordon Brown's role in shaping drugs policy. "He is the first prime minister... that has ever in the history of the Misuse of Drugs Act gone against the advice of its scientific panel," he said.

Nutt has become increasingly frustrated at the government's decision to ignore the evidence provided by the council. Following decisions to reject its advice on cannabis and ecstasy, there have been questions about its purpose.

Richard Garside, director of the CCJS, said it was "important that the council is in a position to give honest and impartial advice without fear or favour". He added: "One has to ask the question: who would wish to chair the council at this point, given the quite blatant political interference it has to deal with?"

Professor Colin Blakemore, professor of neuroscience at Oxford University and former chief executive of the Medical Research Council, said it was vital that government policy was based on evidence. "Nobody rational could possibly want a government based on any other type of policy-making."

His comments were echoed by a spokesperson for the Government Office for Science, part of Lord Mandelson's Department for Business, Innovation and Skills, who said it was "vitally important that scientists are able to give objective and independent advice to ministers". Nutt has argued that all drugs, including alcohol and tobacco, should be ranked by a "harm" index, with alcohol coming fifth behind cocaine, heroin, barbiturates and methadone. According to Nutt, tobacco should rank ninth, ahead of cannabis, LSD and ecstasy.

Lost in last week's furore was Nutt's comments about ketamine, which he suggested in evidence to the Commons home affairs select committee should be upgraded from a class C drug because of the harm it can do to users. Nutt's supporters say his comments were consistent with his argument that drugs policy should be evidence-based.

The home secretary asked Nutt to resign because he had "lost confidence" in his ability to give impartial advice. Johnson also accused Nutt of going beyond his remit and of "lobbying for a change in government policy".

"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 told Nutt in his letter asking him to quit. "As my lead adviser on drugs harm, I am afraid the manner in which you have acted runs contrary to your responsibilities," Johnson added. "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."

A former government chief scientific adviser, Sir David King, also said that Nutt had gone too far . "I do feel that if David was critical of Jacqui Smith and her individual decision-making that's stepping over the line," King told the Today programme. But he added: "I think the government has lacked courage in backing David's committee's advice."

Keith Vaz, who chairs the home affairs select committee, described Nutt's comments as "unwise". Vaz said: "As the country's top adviser on the issue, he is implying to many young people that cannabis is not particularly dangerous."

However, Vaz, one of Labour's most influential MPs, also appeared to have concerns about the nature of Nutt's sacking. "It is important to have an honest and open debate about drugs," Vaz said. "There is a delicate balance to strike between showing leadership in this area and facilitating a scientific debate."

Politicians from other parties expressed shock at the decision. Phil Willis, Liberal Democrat chairman of the science and technology select committee, said he was seeking clarification from the Home Office. "We are going back to what George Bush did when for eight years he put science back into the doldrums because his administration would not take advice except from trusted people," Willis said. "Is that what we want in the UK?"

Evan Harris, another Liberal Democrat member of the science and technology select committee, said: "The political sacking of a distinguished scientist, who is the chair of an independent scientific advisory committee, for the 'crime' of having different views than the secretary of state, is an enormous blow to the credibility of the government's approach to scientific evidence."

There has been speculation that Johnson might be forced to change his request to Nutt to quit if a sufficient number of council members came out in his support.

Last night a former home secretary, Charles Clarke, called for a major review of drugs policy and urged ministers not to "isolate themselves" from science. Clarke said the work of the advisory council had always been "extremely valuable" when he was at the Home Office.

"In 2006 I came to the view that the whole classification system needed to be examined as it was out of time and gave the advisory council a mandate to investigate that and report back to me," Clarke said. "I still think that would be the right course of action."

The science minister, Lord Drayson, who is in Japan on official business, announced on the social networking network Twitter in the early hours of yesterday that he had just heard about Nutt's departure and "I am looking into it", triggering speculation that he is privately concerned about the way relations with the scientific community might be affected.

A spokesman refused to elaborate on his views.

Drugs charities were also critical. "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," said Harry Shapiro, information director at DrugScope, the independent body that tracks the price of street drugs.

Fiona Fox, director of the Science Media Centre, called for independent scientific advisers "to be allowed to communicate their evidence and findings to the media and the public as far away from the political process as humanly possible."

She hoped that lessons would be learned from Nutt's forced resignation. "Those in government who care about independent scientific advice – and there are many – now need to use this crisis as an opportunity to clarify the situation and provide assurances to scientists who are willing to share their expertise on contentious issues that they are free to speak their minds."

The Cannabis debate

2004 David Blunkett downgrades cannabis to Class C on the recommendation of the independent Advisory Council on the Misuse of Drugs. Chief constables support the decision, but Keith Hellawell, the drugs czar, resigns and police organisations later criticise it.

March 2005 Charles Clarke becomes home secretary and announces review of cannabis decision.

January 2006 The ACMD recommends that cannabis should remain as Class C, and reports that cannabis consumption has fallen since reclassification. Clarke accepts the report, and begins an internal review of whether the 35-year-old classification system should be overhauled to be based on better evidence of harm.

May 2006 John Reid becomes home secretary and drops review.

March 2007 Professor David Nutt, then a senior member of the ACMD, publishes research in the Lancet suggesting the classification scheme was flawed and not related to actual evidence of harm caused.

July 2007 New prime minister Gordon Brown announces third review of reclassification, claiming public concern over stronger varieties of skunk.

November 2007 Association of Chief Police Officers says it now believes cannabis should be upgraded.

May 2008 ACMD report confirms advice that cannabis remain in Class C, but home secretary Jacqui Smith overrules it for the first time in its history and moves drug to Class B. Council chair Sir Michael Rawlins says he is stepping down and Nutt succeeds him in October.

February 2009 Second ACMD report, recommending reclassification of Ecstasy, is overruled.

October 2009 Nutt is forced to resign by home secretary Alan Johnson after claiming that evidence on drug harm was being distorted and that the issue had been politicised.