Former Home Secretary Charles Clarke has waded into the escalating drugs classification row, accusing Gordon Brown of ignoring his own scientific advisers.
Mr Clarke insisted the Prime Minister had been wrong to "prejudge" the decision on toughening cannabis laws before experts had even studied the issue.
The intervention came as more members of the Advisory Council on the Misuse of Drugs (ACMD) appeared on the verge of quitting in protest over the sacking of their chairman, Professor David Nutt.
Home Secretary Alan Johnson said Prof Nutt's position had become untenable because he was effectively campaigning against Government policy by calling for cannabis to become a class C drug.
But writing in the Times Prof Nutt warned that the two ACMD members who had already stood down were only the start, and the body could cease to exist altogether. "It seems unlikely that any 'true' scientist will be able to work for this, or future, home secretaries," he insisted.
Conservative leader David Cameron branded the row "very unseemly", and said there had been a "breakdown of confidence" between ministers and advisers. But he backed the Government's policy of maintaining higher penalties for cannabis.
Mr Clarke told BBC Radio 4's Today programme that ministers had to reassure scientists that they would be taken seriously.
"I think the mistake was right at the beginning of (Mr Brown's) premiership, saying he was going to change the cannabis rules before the advisory committee had considered its position. I think that was an error."
Mr Clarke went on: "I think what is most important is the Government moves to reassure the scientific community in general that the advisory committees are there to look at the issues in the round and to give their opinion, and their opinion will be taken seriously.
"I think they are entitled to expect that, if they give their advice, their conclusions will be very seriously considered. If it is being prejudged in a different direction, that doesn't arise and I think that is where the difficulties have arisen here."
