View attachment 13944 Another senior government drugs adviser has quit, hours before ministers were expected to ban a new "legal high".
The resignation of Dr Polly Taylor from the Advisory Council on the Misuse of Drugs has sparked speculation that the ban on mephedrone could be delayed.
Lib Dem science spokesman Dr Evan Harris said Home Secretary Alan Johnson's ban would have to wait until the council was "properly constituted".
The Home Office said it would not speculate on any delay to the ban.
There has been media pressure to ban mephedrone, which has been linked in recent press reports to at least four deaths in the UK.
But the law says any move to ban a drug must follow consultation with the Advisory Council.
The council will make its decision whether the drug should be banned on Monday morning, and Mr Johnson was expected to announce a ban later in the day.
However, the council's meeting will be overshadowed by the resignation of Dr Taylor - the ACMD's veterinary medicine expert whose post is required by law to be filled on the committee.
She said she "did not have trust" in the way the government would treat the council's advice.
Dr Harris said: "The 1971 (Misuse of Drugs) Act is very clear that before the government criminalises thousands of people by banning a drug they must take advice on drug harm and other matters from a legally-constituted advisory council.
"If it is necessary to act urgently to ban mephedrone then, by provoking this resignation by their refusal to respect the scientists who offer advice, the home secretary will now be forced to wait a delay while the council is properly constituted."
A former secretary of the advisory council, Jeremy Sare, told the BBC the council would not be properly constituted until a new vet was appointed, which would take them at least "a few weeks".
A Home Office spokeswoman told the BBC: "We have not received any form of resignation from any member of the advisory council, and we cannot speculate on any issues that could arise on Monday.
"Whatever happens does not change the home secretary's commitment on mephedrone."
In her resignation letter, Dr Taylor told the home secretary she was quitting because she did not have trust in the way the government would treat the ACMD's advice.
"I feel that there is little more we can do to describe the importance of ensuring that advice is not subjected to a desire to please ministers or the mood of the day's press," she wrote.
Mephedrone, a synthetic stimulant, is known by various names, including "M-Cat", and "meow meow".
So far there is no scientific proof that mephedrone has been responsible for any deaths in the UK, and scientists are still trying to work out whether it is harmful on its own or if taken with something else.
However, last week, the government's chief drugs adviser, Professor Les Iversen, strongly indicated that the ACMD would recommend classifying mephedrone as a Class B drug.
Last October, Mr Johnson sacked his chief drugs adviser, Professor David Nutt, saying he had lobbied against government policy.
The sacking led five other members to quit and an urgent review of the committee's working relationship with ministers.
Dr Nutt, who has set up his own rival expert body, has warned that banning mephedrone could be self-defeating and that the evidence supporting a ban wasn't clear.
He has urged the ACMD and ministers to wait for the verdict of an expert European body which is looking at the use of the drug across all EU member states.
"This is a pivotal moment in UK drug policy," said Dr Nutt.
"Given the plethora of 'legal highs' that could follow in mephedrone's wake, the way in which this issue is handled could well set the tone for many years to come."
The Conservatives have called for the law to be changed to allow temporary bans of drugs while the scientific evidence is assessed.