David Cameron today admitted he had done things in his past he “should not have done and regretted” over allegations he smoked cannabis as a schoolboy at Eton.

The Conservative Party leader refused to confirm the allegations, published in a new biography, that he was disciplined for smoking the drug at the private school.

Speaking from outside his constituency home in Dean, Oxfordshire, Mr Cameron said: “I do believe that politicians are entitled to a past that is private and remains private.”

Mr Cameron told reporters he would not be “making any commentary” on what was in today’s newspapers. He said: “I’m not issuing a denial, what I am saying is that I think it’s an important principle that politicians are entitled to a private past.

“Today, I’m a Member of Parliament, I’m someone putting myself forward to be Prime Minister. You are perfectly entitled to come and follow me round, put cameras up my nose, have a good look at me, even come and watch me cook Sunday lunch if you like, but I wouldn’t recommend it.

“But I do think you are entitled to a private past, that is an important principle and one that I’m going to stand by.”

Dressed in a brown round-necked jumper and beige cords, Mr Cameron appeared relaxed about the allegations. After talking to reporters he left his house in the Cotswold village to take his children to church.

If true, the revelations would make Mr Cameron the first leader of a major party and prospective Prime Minister to have been shown to have indulged in illegal narcotics.

Mr Cameron’s refusal to deny the claims could make him vulnerable to his probable rival in the next General Election, Gordon Brown, who has made clear that he has never taken banned substances.

The Tory leader was disciplined for smoking cannabis at Eton when he was 15 years old, sources close to the Cameron admitted this weekend.

Mr Cameron, who has always refused to discuss whether he used drugs, was caught up in a police investigation into drug dealing at the public school. He was hauled before the headmaster, and “gated” — confined to school grounds — after confessing to dabbling with the drug.

Seven pupils were expelled in a major police operation at the school in 1982, involving the drug squad. Pupils’ rooms in Cameron’s boarding house were searched by officers, and two suspects were taken to the police station for interview.

The new biography of the Tory leader claims he and other schoolboys were “snitched on” by pupils who had been dealing in the drug.

Cameron’s confession of smoking what the friend called a “spliff” saved him from greater punishment, despite his refusal to tell masters who else was involved.

The pupils who originally obtained the drugs are said to have bought them either in nearby Slough or from a dealer in Notting Hill, the area where the Conservative leader now lives.

Although Cameron is said to have been shaken by the incident at the time, the book by two journalists claims he went on to take cannabis at university. It says he was an “infrequent and moderate” user.

The disclosure is likely to lead to fresh questions about Cameron’s past. Lord Tebbit, the former Tory chairman, urged him this weekend to be “upfront” about any drug use in the past.

During his campaign for the Tory leadership in 2005, Cameron rode out questions about drug-taking by refusing to comment except to say that MPs had the right to a private life before they came into politics.

His stance won sympathy when aides disclosed that a close member of his family had been a heroin addict. Tebbit, however, described his evasiveness as a “tactical error”.

A senior Labour source commented: “People may dismiss this story as schoolboy pranks, but I think we’ll see in due course that the same casual, dilettante, ‘anything goes’ attitude to drug abuse still persists in Cameron’s inner circle today.”

A number of senior politicians have admitted smoking cannabis, including Charles Clarke and the late Mo Mowlam, the former Labour cabinet ministers.

The Conservative leader’s former headteacher - now Sir Eric Anderson - refused to discuss his fomrer pupil with the book’s authors. But he said: “We would have said, ‘Let’s get the ring leaders’, and if there were others involved, we would have scared them off from doing it again.”

Last month, Mr Cameron said he would consider legalising cannabis for medicinal use if he won power, but ruled out decriminalising it for recreational use. “If you decriminalise, you increase the availability and make it more difficult for parents who are trying to keep their children away from drugs,” he said on his webcameron website.

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