Three more government advisers have resigned after the home secretary's sacking of his chief drugs adviser Prof David Nutt, the BBC has learned.
Dr John Marsden, Dr Ian Ragan and Dr Simon Campbell have quit the Advisory Council for the Misuse of Drugs after two others left earlier this month.
Members of the council had met Alan Johnson seeking reassurance that their independence would not be compromised.
Mr Johnson dismissed Prof Nutt for "crossing a line" into politics.
BBC home affairs editor Mark Easton said the absence of certain experts on the council meant it was "stymied" and effectively was now unable to operate under its current constitution.
He said the remaining 25 members still had outstanding concerns and Mr Johnson was presented with "a big problem".
The Home Office confirmed that the ACMD had to have 20 members to remain quorate, and that six positions had to be filled for the advisory group to function.
The three to resign were psychologist Dr John Marsden, Simon Campbell, a synthetic organic chemist who received a CBE for services to science, and pharmaceutical consultant Ian Ragan.
Earlier the home secretary said he stood by the decision to remove Prof Nutt but wanted to improve relations with the council, saying he would give "due weight" to their views in the future.
The Home Office had published a statement following a meeting between Mr Johnson and the ACMD on Tuesday.
It said: "The discussions were very constructive, and it was agreed that the ACMD would continue discussions with the Home Office and government chief scientific advisors in establishing a way to work collaboratively together into the future with a common purpose of reducing any drug-related harm in the UK."
Free from influence
Clinical director Marion Walker and Dr Les King were the first to resign following the row.
Dr King said he wanted the council to become independent, free from government influence.
Prof Nutt was sacked after calling cannabis less harmful than alcohol and nicotine, and saying it was upgraded to Class B for political reasons.
But Mr Johnson said Prof Nutt had "crossed the line between offering advice and then campaigning against the government on political decisions".
Shadow home secretary Chris Grayling said: "Whilst we backed the original decision, by now I would have expected the home secretary to be able to sit down with other members of the council and rebuild confidence and stability in what they are doing. Quite clearly he has failed to do that."
Liberal Democrat science spokesman Dr Evan Harris said: "The fact the resignations come after Alan Johnson met the ACMD demonstrates he just doesn't get it when it comes to the importance of respecting the academic freedom and integrity of independent, unpaid, science advisers."
21:01 GMT, Tuesday, 10 November 2009