Dr Polly Taylor is the sixth member of the Advisory Council on the Misuse of Drugs to resign since the sacking of the chairman, Professor David Nutt, last year.

The move will come as a blow to Mr Johnson who is due to meet with his chief drugs advisor, Professor Les Iversen and is understood to be likely to accept his advice that the drug be placed in Class B.

Dr Taylor's departure could derail plans to pass a ban through Parliament before the expected start of the general election campaign. The Advisory Council on the Misuse of Drugs (ACMD), which Prof Iversen chairs, is required to present a full report on the dangers of mephedrone before a ban can come into force. But this is unlikely to happen until after Easter, according to Home Office sources.

Once the report is passed on, the change in the law would have to be approved by both houses of Parliament and the Privy Council. Critics alleged there was a delay in banning the drug, which has been linked to a series of deaths in recent weeks.

Liberal Democrat home affairs spokesman Chris Huhne said work by the ACMD on the dangers of mephedrone was put back because of resignations from the council, which included the expert in charge of research on the drug. In a letter to the Home Secretary, Dr Taylor said she feared the panel's advice was not being treated independently. She wrote: "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."

Parliamentary rules require that before a drug can be reclassified the government must consult a "properly constituted" ACMD, which should include a vet among its members. The families of victims have called for a ban on mephedrone, which is known as Plant Food, or Miaow Miaow and sold on the internet as a "legal high".

Last week 24-year-old Lois Waters from Norton, North Yorkshire became the latest person to die after taking the drug. Her death followed those of Louis Wainwright, 18, and Nicholas Smith, 19, in Scunthorpe.