The shocking claims were made in a new biography, Call Me Dave
David Cameron once put a "private part of his anatomy" into a dead pig's mouth during a bizarre initiation ritual, it has been claimed.
The Prime Minister carried out the act as a student a Oxford University, according to a claim made in a new biography.
The story was recounted to the authors by a contemporary of Mr Cameron who went on to become an MP.
He reportedly said the pig's head was resting on the lap of a member of the Piers Gaveston Society – a dining club at Oxford – while Cameron carried out the act.
He even claimed another member of the group has photographic evidence of the alleged act.
But the individual who is said to own the picture did not respond to approaches by the authors.
The shocking allegation features in Call Me Dave, a biography of Mr Cameron co-written by Tory peer Lord Ashcroft and the journalist Isabel Oakeshott.
Mr Ashcroft said: "Perhaps it is a case of mistaken identity. Yet it is an elaborate story for an otherwise credible figure to invent.
"Furthermore, there are a number of accounts of pigs' heads at debauched parties in Cameron's day."
The alleged act echoes the first episode of the drama series Black Mirror, in which the fictional Prime Minister has sex with a pig live on TV.
The alleged act echoes the first episode of the Black Mirror
The Prime Minister studied Philosophy, Politics and Economics at Oxford
The book also claims he was part of a "dope smoking group" called the Flam Club at university, according to the Daily Mail.
James Delingpole, a friend of Mr Cameron's at Oxford, said they smoked cannabis together while listening to the rock band Supertramp.
He said: "My drug of choice was weed – and I smoked weed with Dave."
Mr Cameron studied Philosophy, Politics and Economics at Oxford's Brasenose College between and 1985 and 1988.
A Downing Street spokesman refused to comment when contacted by Express.co.uk.