Dr Les King, a chemist, quit the Advisory Council on the Misuse of Drugs Advisory Council on the Misuse of Drugs and said that the Home Secretary had denied the chairman’s right to free speech when he sacked him.

He was followed by Marion Walker, a pharmacist, who is clinical director with the substance misuse service at the Berkshire Healthcare NHS Foundation Trust.

Professor David Nutt, chairman of the council, was dismissed after saying that cannabis was less harmful than alcohol or nicotine and had been reclassified for political reasons.

Dr King, who became a full member of the council last year, said that the Government’s attitude to the panel had been shifting in recent years 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.”

Dr King said he believed that the panel needed to become “free from government interference” in the same way as the National Institute for Health and Clinical Excellence (NICE), the organisation that 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.

Professor David Nutt, who was dismissed as chairman of the Advisory Council on the Misuse of Drugs (ACMD) yesterday after he claimed that illegal drugs such as cannabis and ecstasy are less dangerous than alcohol and tobacco, is warning that more senior scientific advisors are set to walk out over the row over drug classification.

He was asked to resign by Home Secretary Alan Johnson, who claimed he has "lost confidence" in the expert’s political impartiality.

Today, Professor Nutt claimed Gordon Brown’s government was alienating themselves from the scientific community appointed to advise them.

He said: "Gordon Brown makes completely irrational statements about cannabis being 'lethal', which it is not.

“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.”

Professor Nutt, who has been a vocal critic of the Government’s reclassification of cannabis from Class C back to Class B, claims that many of the other 30 members of the drugs advisory council could also be set to resign.

“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.”

“I wouldn’t be surprised if some of them stepped down. Maybe all of them will. I'm not prepared to mislead the public about the harmfulness of drugs like cannabis and ecstasy.

"I think most scientists will see this as a further example of the Luddite attitude of this Government, and possible future governments.”

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.