He also supported calls for ecstasy to be downgraded from the class-A status it shares with cocaine and heroin and said it would be "disappointing" if radical options on the law on cannabis were not looked at. His remarks will shock many Tories MPs who have traditionally taken a hard line on drug possession and use. The leadership contender said he favoured " fresh thinking and a new approach" towards drugs policy and said "we have to let 1,000 flowers bloom and look at all sort of different treatment models" for heroin addicts.

Ann Widdecombe, the former Home Office minister who is supporting Kenneth Clarke for the Tory leadership, criticised Mr Cameron's views and said that legalising drugs would only encourage use. "This is a grossly misled view that will have very damaging consequences for society," she said. "Most Conservatives would make the case that legalisation is misguided. If you legalise hard drugs you would effectively be making the state give first-time users their first experience. "It's just not an option. And the World Health Organisation is against it."

Legalisation campaigners welcomed Mr Cameron's stance, saying he recognised that current policy, which involved criminalising users, had failed. " David Cameron deserves our utmost respect and admiration for refusing the 'war on drugs' rhetoric in calling for a discussion of legalisation with the UN body that oversees global prohibition," said Danny Kushlick, the director of the Transform Drug Policy Foundation. He added: "Too many politicians support the status quo because of careerism."

The Conservative leadership contender voted, when he was a member of the Home Affairs Select Committee, for the UN body on drugs policy to look at whether to legalise and regulate the drugs trade. He opposed his Tory colleague, Angela Watkinson, who tried to block the call to initiate talks in the UN and voted against her with Labour and Liberal Democrat MPs. He called on the Government to "initiate a discussion" within the UN about "alternative ways - including the possibility of legalisation and regulation - to tackle the global drugs dilemma".

In its report on drugs policy, published three years ago, the select committee backed the decision by the then home secretary, David Blunkett, to downgrade cannabis from class B to class C, a decison which is being reviewed after concerns that stronger forms of cannabis, such as skunk, are in more widespread use. The report also supported creating safe "shooting galleries" for heroin addicts.

A spokesman for Mr Cameron said he was not making the case for legalising drugs but that he supported keeping "an open mind" about dealing with drug abuse. "Drugs strategy has been failing so it is important politicians can keep an open mind about how to deal with it. He isn't saying anything should be done. He is saying options should be considered."

Leadership contenders' views

DAVID CAMERON "Politicians attempt to appeal to the lowest common denominator by posturing with tough policies and calling for crackdown after crackdown. Drugs policy has been failing for decades."

KEN CLARKE "The real answer lies not in changing the law but in... one joined-up policy involving every department and agency - from crop substitution to better drug rehabilitation."

DAVID DAVIS "Drugs fuel crime. The fact that an ecstasy tablet can be bought for less than a can of Coke is a shocking indictment of Labour's absolute failure to tackle the scourge of drugs."

SIR MALCOLM RIFKIND " The move to downgrade cannabis was wrong. The Government retained possession as a criminal offence but it could not be treated as a crime. That makes the law look foolish."

DR LIAM FOX Declined to comment