Excerpts of an unauthorised biography of British Prime Minister David Cameron have revealed some startling and bizarre allegations about the Conservative leader.

Among the unusual claims is an allegation that Mr Cameron placed one of his “private parts” into the mouth of a dead pig while at university, according to the Daily Mail.

The authors of Call Me David, Michael Ashcroft and Isabel Oakeshott, allege the incident occurred as part of an “outrageous initiation ceremony”, at Oxford University.

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The story was told to Mr Ashcroft and Ms Oakeshott by someone who witnessed the alleged act and the book also suggests a photo of it exists.

However, when the authors approached the individual said to have the photo, they did not respond.

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 saga has even spawned a satirical Twitter account:

Look, it’s embarrassing for me, too.

— Camerons Pig (@CameronPig) September 20, 2015

Mr Ashcroft was once an “integral” figure to Mr Cameron’s Tory party, helping save it from financial ruin.

It has long been rumoured that Mr Ashcroft and Mr Cameron had suffered a falling out.

Mr Ashcroft writes in the book’s preface: “Long after he [Mr Cameron] became Prime Minister, the impression persisted that he was more interested in holding the office than in using its power to achieve anything in particular.

“His laissez-faire approach can create the impression that he is insufficiently concerned by results, and more than once he has appeared so relaxed that he has only stirred to avert disaster at the last minute. But my own particular beef with him is more personal.”

Mr Ashcroft also writes of a conversation had with Mr Cameron in 2009 about “how we could delay revealing my tax arrangements until after the election”.

A friend of Mr Cameron’s also tells the authors how the pair would listen to 1970s rock music and smoke drugs in university.

“My drug of choice was weed – and I smoked weed with Dave,” James Delingpole said.

The dramatic headline splash of the Daily Mail edition reads: “Drugs, debauchery and the book that lays Dave bare”.