Social media users have barely been able to contain their delight at claims in a new biography that British Prime Minister David Cameron put "a private part of his anatomy" into the mouth of a dead pig at an initiation ceremony while he was a student at Oxford University.

The allegation was published in Call Me Dave: The Unauthorised Biography and attributed to "a distinguished Oxford contemporary" who is himself an MP, co-authors Lord Michael Ashcroft and Isabel Oakeshott said.

The act was allegedly performed at an event hosted by the notorious Oxford dining club, the Piers Gaveston, named in honour of the supposed lover of King Edward II of England.

The secret society, which also claims actor Hugh Grant and the Mayor of London Boris Johnson among its former members, is known for its debauched parties, sexual excesses and bizarre entertainment.

Ashcroft and Oakeshott claim they have tried to contact an individual who allegedly has photographic evidence of the act, but the person has refused to respond.

"My co-author Isabel Oakeshott and I initially assumed this was a joke. It was therefore a surprise when, some weeks later, the MP repeated the allegation," they wrote for Britain's Daily Mail, which began serialising the biography on Sunday night.

The MP repeated the claim a third time several months later, saying the  pig's head "had been resting on the lap of a Piers Gaveston society member while Cameron performed the act", Ashcroft and Oakeshott wrote.

"Perhaps it is a case of mistaken identity. Yet it is an elaborate story for an otherwise credible figure to invent."

Downing Street has not responded to the claim.

David Cameron engaged in obscene act with pig's head when a student, book claims