Britain's "Daily Mail" newspaper published the extracts on Monday, with reactions on Twitter focusing on the allegation that Prime Minister David Cameron took part in a particularly outlandish dinner club initiation ceremony while a student at Oxford.

The unauthorized biography, "Call Me Dave", was co-written by Michael Ashcroft, a former Cameron ally who turned against the prime minister.

It includes a number of new, but unremarkable claims of his time at university; that the future prime minister enjoyed smoking marijuana while chatting with close friends and listening to Supertramp.

However, one story - involving a dead pig - stood out from all the others. The extract quotes one of Cameron's university contemporaries - apparently now a member of parliament - as saying that the young Cameron had once "inserted a private part of his anatomy into the animal's mouth."

Unwanted exposure

The unnamed source, who said he had seen photographic evidence of the act, was said to have repeated the claim on several occasions. The deed was said to have been part of an initiation ceremony for Oxford's notorious Piers Gaveston dining society.

The Daily Mail, a traditional ally of the Conservative Party, featured the extract on its front page, touting the biography as "the book that lays Dave bare."

Ashcroft appeared pleased with the coverage.

The prime minister's Downing Street office said it would not "dignify" the claims, seeking to portray the book and its contents as a case of sour grapes. Ashcroft has admitted that he had a "beef" with Cameron after not being offered a senior cabinet job when the conservatives were elected as Britain's biggest party in 2010.

Twitter erupts with jibes

Twitter users reveled in pig puns about the alleged incident, including the "Sunday Times" deputy political editor, James Lyons.

Photos and memes - featuring such popular porcine characters as Peppa Pig and Miss Piggy - were gleefully shared, while mass-circulation daily newspaper "The Sun" made use of a picture taken while Cameron visited a farm in his Oxfordshire consituency of Whitney.

A club for the privileged few

Cameron is long known to have been a member of another dubious society - the notorious Bullingdon Club - a waywardly boisterous drinking club for the Oxford University's super-rich young men.

The group's bespoke uniform - navy tailcoats, mustard waistcoats and sky-blue bow ties, would cost thousands of pounds, meaning that membership of the club was out-of-reach for ordinary students. Among the PM's Bullingdon Club contemporaries were British Chancellor of the Exchequer George Osborne and Mayor of London Boris Johnson - both tipped as possible successors to Cameron at the head of the Conservative party.

Members are reputed to have held dinners that revel in excessive drinking and mindless destruction, punctuated with disdainful tirades against the poor.

The club's former members include British arch-imperialist Cecil Rhodes, abdicated English king and alleged Nazi-sympathizer Edward XVIII, and the father of wartime Prime Minister Winston Churchill.