The Tory leader, who has refused to answer media questions about whether he used drugs before entering politics, ruled out a wider legalisation of cannabis for recreational use.

Answering questions on his "webcameron" website, he said: "If it can be proved that there are real benefits, medicinal benefits and scientific evidence for it, I would be relaxed about that. My decision would be to licence it if it could be proved to have benefits."

The pledge will be welcomed by campaigners who claim that cannabis can ease the symptoms of illnesses such as multiple sclerosis, but it may worry Tory traditionalists.

Mo Mowlam, the former cabinet minister who died in 2005, fought for cannabis to be made legal for medical use. Although the Home Office set up investigations into the drug's possible benefits, the impetus appears to have stalled.

Mr Cameron will launch a new Tory health policy today vowing to sweep away many of the targets Labour has imposed on the NHS.

The Tories will call for all GPs to take control of budgets, currently held by primary care trusts, and be rewarded for improving outcomes. GP fundholding was used by the Tory government before 1997, but it was scrapped by Labour.

The Tories say outcomes matter more than targets and that Labour's regime distorts priorities. Maximum waiting times set by the Government are becoming minimum waits, they argue.