Eric Carlin became the latest member of the Advisory Council on the Misuse of Drugs (ACMD) to resign, in what he said was a protest in the way mephedrone had been criminalised.
He said the decision taken earlier this week was "unduly based on media and political pressure".
Mr Carlin, 47, is the latest member of the ACMD to resign following the sacking of former chairman Professor David Nutt.
He said he had grown disillusioned with the ACMD's "lack of interest" in prevention and early intervention with young people.
Earlier this week Home Secretary Alan Johnson announced the dance drug mephedrone would be banned within weeks.
The legal high, linked to up to 25 deaths in England and Scotland, will be banned and made a Class B drug.
Mr Carlin, speaking from Brussels, said he believed the decision to rush through the ban had been politically motivated in order for the Government to look tough prior to the election.
He tendered his letter of resignation to the Home Secretary yesterday.
It reads: "We had little or no discussion about how our recommendation to classify this drug would be likely to impact on young people's behaviour.
"Our decision was unduly based on media and political pressure."
He added: "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."
Danny Kushlick, director of the Transform Drug Policy Foundation said: "This latest resignation is because Carlin recognises that criminalisation causes harm for young people in direct contradiction to the Government's stated intention."
Mr Carlin said far more consideration and debate has been needed into how young people use the drug.
He explained: "We've not properly considered it, not assessed how young people use it."
Prof Nutt was sacked last year after saying ecstasy was less harmful than alcohol.
Earlier this week, Dr Polly Taylor a long-standing member of the ACMD, quit just hours before Mr Johnson's mephedrone announcement.
A spokesman for the ACMD said: "We can confirm that Mr Carlin has tendered his resignation to the Advisory Council on the Misuse of Drugs."
A Home Office spokesman added that the ACMD would still continue its work.