The Advisory Council on the Misuse of Drugs is "in deep trouble", according to the latest expert to resign from it over the criminalisation of mephedrone.
Eric Carlin said ministers had pledged to ban the drug to appear to be "acting tough" in the run-up to the election.
He said experts were being ignored and the council was "not doing its job".
The council has recommended that the substance, linked to at least four UK deaths, and other so-called "legal highs" be classified as Class B drugs.
Mr Carlin told the BBC that when plans to ban mephedrone were announced: "The home secretary was briefed and doing press conferences before we'd even considered all our recommendations.
"If you have an expert advisory committee you have to deal with them as experts. I hope any incoming government will look at the role of external advisors, I think it's incredibly important.
"We need to fundamentally re-frame this, and deal with it as a public health issue, not primarily as a criminal justice issue.
"There are more young people using illegal drugs now than ever did before, more young people drinking than ever did before.
"What we fundamentally need to do is get to the root causes of why is it that our 14, 15-year-olds are getting off their faces?"
Mr Carlin said he did not think the council should be scrapped, but said it was essential it was a genuinely independent body and its decisions were based on science not politics.
But Professor Neil McKeganey, who advises the Scottish government about drugs, said the council should be disbanded.
He told BBC Radio 5 Live: "This council is not working in the way in which it was intended. One is seeing serial resignations, week by week. I think to most people's judgement, the relationship between the ACMD and government has broken down".
Mr Carlin, 47, a former chairman of the English Drug Education Forum, is the seventh member of the body to resign following the sacking of former chairman Professor David Nutt.
In his letter to Home Secretary Alan Johnson, he wrote: "As well as being extremely unhappy with how the ACMD operates, I am not prepared to continue to be part of a body which, as its main activity, works to facilitate the potential criminalisation of increasing numbers of young people".
However another member of the ACMD told the BBC that they felt "properly consulted" and there were at least four meetings where mephedrone was discussed by the council or its expert committees.
The member, who does not have a scientific background, said they "did not feel under the slightest pressure from politicians or the media" when it came to making the decision to recommend that mephedrone should be classified as a Class B drug.
Mr Carlin's resignation comes just days after another adviser, Dr Polly Taylor, quit the ACMD for similar reasons.
Professor Nutt, who was sacked by Mr Johnson in October 2009, said he was not surprised by Mr Carlin's decision.
The Home Office called Mr Carlin's resignation "regrettable".
A spokesman said: "However it does not impact on our plans to ban mephedrone and the other substances as soon as parliamentary time allows.
"The home secretary has full confidence in [chairman] Professor [Les] Iversen and the rest of the ACMD committee".
The home secretary will ban mephedrone and the other substances within weeks, following the advice of the ACMD.
Class B drugs, which include cannabis and amphetamine sulphate, carry a maximum sentence of five years in prison for possession or 14 years for supply.
BBC News
