Two more drug advisers to the British government have stepped down amid an increasingly heated debate over the country's marijuana policy.

The resignation of chemist Les King and clinical director Marion Walker from Britain's Advisory Council on the Misuse of Drugs follows the ouster of the group's chairman David Nutt, who was forced to step down after speaking out against the government's decision to tighten restrictions on marijuana.

King has warned that the group, which has advised the British government on drug issues for nearly four decades, could fall apart.

"It's being asked to rubber stamp a predetermined position," he said, explaining that British ministers were putting inappropriate pressure on scientists to make decisions on drugs for political reasons.

He said others could follow him by leaving the council, a move that would be deeply embarrassing for the government.

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

The debate has its roots in the British government's decision to officially reclassify marijuana as a more dangerous kind of drug. Previously marijuana was considered a "Class C" drug, whose possession could carry a sentence of up to two years in prison.

But the government recently upgraded the drug to "Class B," meaning that marijuana has joined amphetamines and Ritalin, without a prescription, as substances whose possession can result in up to five years in prison. Cocaine and Ecstasy, for instance, are classified as "Class A" drugs, possession of which could lead to seven years in prison.

The move went against the advice Nutt and his council. The professor of psychopharmacology has consistently argued that drugs such as marijuana and Ecstasy are far less dangerous than alcohol and that the restrictions placed on them should be proportional to their potential harm.

Britain's Home Office rejected his advice last year, calling the scientific evidence uncertain and saying that a message needed to be sent to marijuana users that possessing the drug is a serious crime.

Although Nutt's position has long been known, Britain's Home Office Secretary Alan Johnson forced him to resign after he revisited the issue in recent media interviews and accused the government of distorting scientific evidence.

Johnson he respected Nutt's views but the adviser had "crossed the line."

"You cannot have a chief adviser at the same time stepping into the public field and campaigning against government decisions," he told Sky News television in a sometimes testy interview. "You can do one or the other, you can't do both."

Nutt's dismissal has caused dismay in the scientific community.

Scientist Robert Winston, a lawmaker with Johnson's governing Labour Party, said Nutt had made a "very reasonable" point about the relative dangers of illegal and legal drugs, telling BBC radio that he was disappointed by Johnson's actions.

"I think that if governments appoint expert advice they shouldn't dismiss it so lightly," he said. "I think it shows a rather poor understanding of the value of science."

The council has 31 members, three of which have now stepped down. Johnson said he would be meeting with the remaining members shortly.

As for Nutt, he said he would continue to speak out on the issue.

"I cannot stand by and see the truth distorted for political reasons," he told Sky.