Another senior government drugs adviser has quit, hours before ministers are expected to ban a new "legal high".
Dr Polly Taylor said she did not trust the government's use of advice from the Advisory Council on the Misuse of Drugs. She is the sixth member to quit.
On Monday the body will give the home secretary its advice on mephedrone.
It is unclear if Dr Taylor's resignation will affect the status of the expected ban on the drug, linked in the media to four deaths in the UK.
The council's meeting on Monday could be overshadowed by the departure of Dr Taylor - the ACMD's veterinary medicine expert whose post is required by law to be filled on the committee.
A Home Office spokeswoman said she would not speculate on any delay to the ban.
 |  There is little more we can do to describe the importance of ensuring that advice is not subjected to a desire to please ministers  |
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, there is widespread expectation that Home Secretary Alan Johnson will announce a ban on the drug before the end of the day because of the risks it poses.
Last week, the government's chief drugs adviser, Professor Les Iversen, strongly indicated that the council would recommend classifying mephedrone as a Class B drug.
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 its 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 FACTS Effects similar to amphetamines and ecstasy Sold as a white powder, capsules and pills or can be dissolved in liquid Often sold online as plant food marked "not for human consumption" Completely different to methadone, used to treat heroin addicts Reported side-effects include headaches, palpitations, nausea, cold or blue fingers Long-term effect unknown Currently legal to buy and be in possession of the powder, but against the law to sell, supply or advertise the powder for human consumption Already illegal in Israel, Denmark, Norway and Sweden |
Last October, Mr Johnson sacked his chief drugs adviser, Professor David Nutt, saying the ACMD chairman 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 is not 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.
