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.