There were predictions today of mass resignations from the country's drugs advisory panel in protest at Alan Johnson's decision to axe its chairman for criticising the Government's policy.

At least two members of the Advisory Council on the Misuse of Drugs (ACMD) have quit since Professor David Nutt was sacked by the Home Secretary on Friday in a row that is threatening the panel's future.

Writing in the Times today, the professor warned more of the remaining 28 members could leave, saying: "It seems unlikely that any 'true' scientist will be able to work for this, or future, home secretaries.

"My sacking has cast a huge shadow over the relationship of science to policy. Several of the science experts from the ACMD have resigned in protest and it seems likely that many others will follow suit.

"This means the Home Office no longer has a functioning advisory group, which is very unfortunate given the ever-increasing problems of drugs and the emergence of new ones."

Fellow panel member Dr Les King resigned yesterday, saying Mr Johnson had denied Prof Nutt his right to free speech and called for the advisory panel to become truly independent from politicians.

A second member, pharmacist Marion Walker, is also understood to have quit.

Dr King said the Government's attitude to the panel has shifted and home secretaries now had a "pre-defined political agenda" when they asked for its expert advice.

"It's being asked to rubber stamp a pre-determined position," he said.

"If sufficient members do resign, the committee will no longer be able to operate."

Writing in the Guardian today, Mr Johnson again explained his rationale for the sacking.

"Professor Nutt was not sacked for his views, which I respect but disagree with," he said.

"He was asked to go because he cannot be both a government adviser and a campaigner against government policy."

Referring to Prof Nutt's previous comparison between the risks from horse-riding and taking ecstasy, he added: "There are not many kids in my constituency in danger of falling off a horse - there are thousands at risk of being sucked into a world of hopeless despair through drug addiction."

The professor reiterated his view that ecstasy and cannabis - class A and B drugs respectively - were not in the correct class and that all drugs should be ranked by a "harm" index.

"It is imperative that that the classification of drugs truly reflects their harms, otherwise injustices may occur and the educational message be undermined," he wrote.

"Scientific inquiry into drug harms must also be honest and accurate so that the best quality evidence is available to the experts and government.

"Legal drugs such as alcohol and tobacco are as harmful as many illegal drugs and currently score highly on our ranking list."

Prof Nutt has said alcohol should rank fifth in the harm index behind cocaine, heroin, barbiturates and methadone. Tobacco should rank ninth, ahead of cannabis, LSD and ecstasy.

He suggested a way forward, saying: "Create a truly independent advisory council. This is the only realistic way out of the current mess."

He also admitted yesterday that more than one of his four adult children had confessed to taking drugs.

But the scientist told the Mail on Sunday that he believed alcohol posed a greater risk to young people and that was where the "big effort" should be concentrated.

Prof Nutt, who has four children aged 18 to 26, said alcohol was "cheap, readily available and accepted" but could lead to "risky behaviour".

Dr King worked for the Forensic Science Service (FSS) for 30 years and spent 10 years as head of the Drugs Intelligence Unit before his retirement in 2001.

He believes the panel needs to become "free from government interference" in the same way as the National Institute for Clinical Excellence (Nice), the organisation which advises on medicines and clinical practice.

"I don't see why drugs can't be done the same. It can be totally depoliticised.

"It's all about harm. It's a scientific issue," he said.

Dr King also said the Government had "a right" to reject the panel's advice but added that the attitude towards the panel had changed "very recently".

"I suppose it goes back to 2002 with (then Home Secretary) David Blunkett who was minded to reclassify cannabis downwards.

"He made it clear to us his wishes. The council supported that.

"In that situation it was something that the council readily agreed to. That wasn't too worrisome but that precedent then continued."