The home secretary has criticised Conservative leadership contender David Cameron's call to downgrade ecstasy from a class A drug.
David Cameron says he wants ecstasy downgraded
Charles Clarke said such a policy would be "irresponsible", as there was "more to learn" about the harm it could do.
Mr Cameron has said a "realistic and sensible" approach is needed to combat the drugs problem.
Tory members have started voting on whether Mr Cameron or rival David Davis should become the next party leader.
Ballot papers were sent out on Friday, leaving the 300,000 members a month to decide.
Mr Davis, the shadow home secretary, said Mr Cameron was "leading at the moment" but that his own "highly principled approach" could succeed.
But a spokesman for shadow education secretary Mr Cameron said he would appeal more to the wider electorate.
During a live TV debate on Thursday, Mr Cameron, who has repeatedly been asked whether he took drugs at university, suggested that ecstasy could be downgraded, to help make policy more "credible" to young people.
But on Friday Mr Cameron's spokesman said the issue would be debated if he became leader, but added: "It would be a matter for the party to decide."
Mr Cameron later told a meeting in Winchester, Hampshire: "What people want is a realistic and sensible policy that gets to the bottom of the drugs problem."
He wanted to see ex-addicts going into schools to talk about the dangers of drugs and tell of the difference between substances such as heroin and ecstasy.
Mr Cameron said that this was a "tough" drugs policy, adding: "Frankly I do not care what other people think about it."
Mr Clarke said it would be irresponsible to reclassify ecstasy, as it could and did kill unpredictably .
There was still more to learn about the harm it caused, a spokeswoman added.
Odds on Mr Davis shortened for the first time since October's Conservative party conference after the BBC Question Time debate but Mr Cameron remains the bookmakers' favourite.
The candidates have begun a nationwide series of hustings, but most of the 300,000 members are expected to return their ballot papers well before the 5 December deadline.
A Cameron campaign spokesman hit back at suggestions Mr Davis had got the better of his younger rival in the televised contest, saying "David Cameron beat him on most questions."
He claimed Mr Davis had come into the debate looking "hopeless" and "incompetent" and had targeted his message at core Conservative supporters, while Mr Cameron tried to speak to the wider electorate and a "new generation" of voters.
During Thursday's TV debate, Mr Davis claimed his rival was focusing on spin rather than substance, but Mr Cameron countered by saying Mr Davis was setting policies to win headlines.