Conservative leader David Cameron took drugs while he was a pupil at Eton College, a new biography has claimed.
The book, serialised in the Independent on Sunday, says Mr Cameron, then aged 15, was one of several boys caught smoking cannabis at Eton.
He confessed and was grounded. Some of the other boys were expelled.
A Conservative Party spokesman said: "David has always maintained politicians have a right to a private life before they come into politics."
He pointed out that the alleged incident happened almost 25 years ago. Mr Cameron, now 40, has so far made no comment.
The book - Cameron, The Rise Of The New Conservative - will also be serialised in the Mail on Sunday next month.
Both papers report that school authorities called the police to investigate drug use among the pupils.
Because he had smoked cannabis and not sold it, Mr Cameron was not expelled or suspended like several other boys, the book claims.
Instead, he was fined, grounded for two weeks and given the school's traditional punishment of a "Georgic" - copying out hundreds of lines of Latin poetry, according to the book.
Throughout his leadership campaign in 2005, Mr Cameron declined to answer questions about drug taking when they were put to candidates.
He repeatedly stressed he had a right to a private past and refused to answer them.
Mr Cameron was initially asked at a fringe meeting at the 2005 Conservative party conference if he had ever taken drugs.
He told the meeting he had had a "typical student experience", later adding: "I did lots of things before I came into politics which I shouldn't have done. We all did."
Later that same year on BBC One's Question Time, he said everybody was allowed to "err and stray" in their past.
He told the audience he would not bow to a "media-driven agenda" to "dig into politicians' private lives".
Former Conservative Party chairman Lord Tebbit told BBC News 24 the claims would not do Mr Cameron much good with Tory activists.
"On the whole, I've always thought that it was better to be pretty honest about things," he said.
"Because, sooner or later the truth of the matter tends to come out, and it's always better to have brought it out yourself rather than have somebody else bring it out."
Conservative spokesman on Rural Affairs, Peter Ainsworth, said he saw little relevance in the story.
"I frankly don't give a monkey's... it's simply not relevant to what we're doing today with the Conservative Party or to British politics."
He later told BBC News 24: "It's not the sort of story you necessarily want to come out but on the other hand it's something that happened to a schoolboy 25 years ago.
"Most people looking at this and being fair minded people will think 'yes, it's not something I'd be proud of if I'd done or my son had done but there it is, it's a long time a go and people make mistakes'."
However, the Mail on Sunday's political editor, Simon Walters, told BBC Radio Five Live some voters would want reassurances that Mr Cameron would not legalise cannabis.
Last month, Mr Cameron said he opposed making cannabis legal but would be "relaxed" about legalising it for medicinal use if there was evidence of its health benefits.
More than 10 million adults in the UK have used cannabis, according to drugs charity, Drugscope.
Its chief executive, Martin Barnes, said cannabis was not harmless but blaming a generation who used the drug at some time "serves no useful purpose".
"People ready to throw stones on this issue should be aware of the glass houses of their friends, relatives or colleagues," he said.