Party chiefs insisted the revelation that he was disciplined for taking the drug would not affect the voters' view of the Tory leader. But they fear it could reignite the speculation about drug-taking that dogged his leadership campaign.

In a brief statement, Mr Cameron said: "Like many people, I did things when I was young that I shouldn't have done and that I regret. But I do believe that politicians are entitled to a past that remains private."

He did not comment directly on the disclosure in The Independent on Sunday that the Eton authorities called in the police in 1982 amid rumours of drug-taking at the exclusive public school. Seven pupils who sold cannabis were eventually expelled. After being named by another boy, Mr Cameron admitting using the drug, but escaped the most serious punishment because he only smoked it, rather than traded in it. He was "gated" - confined to school grounds - for two weeks, fined and ordered to copy out hundreds of lines of Latin verse.

Although he largely abstained from drug use after the incident, a friend recalls him having "occasionally a joint or something" during his Oxford University days.

The schoolboy escapade nearly 25 years ago emerged in a book by the Independent on Sunday journalists James Hanning and Francis Elliott. As they researched the future Tory leader's childhood, their interest was aroused by the number of fellow pupils who did not want to discuss their school days. His office was irritated by the eventual disclosure of his youthful drug-taking, but was in no position to deny it.

Mr Cameron could be vulnerable on the issue to Gordon Brown, his probable rival at the next election, who has made clear he has never taken illicit drugs.

There were suggestions at Westminster last night that Labour was preparing to exploit the subject if there were more serious revelations about others in Mr Cameron's inner circle.

Tory traditionalists who do not share his socially liberal instincts are also likely to be dismayed by revelations of their leader's drug-taking past. However, party sources are confident the vast majority of voters - particularly younger ones - will take a more relaxed attitude.

William Hague, the former Tory leader, told BBC1's Sunday AM the disclosure made no difference to him. "I can tell you, I work with him every day and of all of us - and there have been quite a few of us - who have led the Tory Party over the past 10 years, he is the best. He is an all-round effective politician. He is the best positioned to take the Conservative Party into government.

"We all did things that we regret and it is one of those things, I suppose."

One of his closest allies, George Osborne, the shadow Chancellor, told BBC1's Politics Show: "It's not been denied by David, but he's also said that we are not in the business of saying that politicians can't have a private life before they come into politics."

Oliver Letwin, chairman of the party's policy review, dismissed suggestions the publication of photographs of him in a £1,000 tailcoat at Oxford's Bullingdon Club would damage his image.

He asked: "Are people allowed to grow up? I hope they are. Are people allowed to try to learn how to behave and how to live in the way they choose? I hope they are. I think we all are."