Professor Les Iversen, the chairman of the Advisory Council on the Misuse of Drugs (ACMD), said his personal view was mephedrone was "amphetamines by another name".

In evidence to the Home Affairs Committee, he said any deaths linked to the drug were a "tragedy", but he defended the need to conduct research into the effects of mephedrone - known as M-Cat or Miaow Miaow - before it was banned.

Addressing MPs, he said: "I am not here to give my personal views... but as a pharmacologist these drugs are amphetamines by another name and I know that amphetamines are harmful. I think you can deduce my conclusions from that."

Amphetamines are currently a Class B drug along with cannabis. Putting mephedrone in Class B would mean carrying the drug would be punishable with a jail term of up to five years and dealing it with up to to 14 years in prison.

The Committee's chairman Keith Vaz said he would be writing to Home Secretary Alan Johnson to complain about the delay in banning mephedrone.

He said: "We will be writing following this session to the Home Secretary about these matters. We just think the delay is most unsatisfactory given the dangers that are inherent (in taking mephedrone)."

The ACMD is due to present its report on mephedrone to ministers on Monday afternoon.

At that stage ministers are likely to indicate that they are in favour of a ban, but it could be many months before a ban comes into force.