21/09/2015 11:49 BST | Updated 21/09/2015 12:59 BST

David Cameron Pig Allegation Journalist Says Story Is 'Colourful And Made Us Laugh' As She Dismisses 'Revenge Job' Claim

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LONDON, ENGLAND - NOVEMBER 10: Prime Minister David Cameron listens to a speech during the the Lord Mayor's Banquet at The Guildhall on November 10, 2014 in London, England. The Lord Mayor of London, Alan Yarrow, is hosting the annual Lord Mayor's Banquet in London's Guildhall which will feature speeches from the Prime Minister and the Archbishop of Canterbury. Alan Yarrow was recently elected 687th Lord Mayor of the City of London, a role that has been in existence since 1189. (Photo by Carl Court/Getty Images)

The alleged story about a "debauched" ritual practiced by David Cameron at an Oxford University club was published because it "made us laugh", one of the authors of the controversial book including the claim has said.

Downing Street today moved to undermine claims included in an unauthorised biography of the Prime Minister, Call Me Dave, which is being serialised by The Daily Mail.

Specifically, it details an an initiation ceremony at the Piers Gaveston dining club where Mr Cameron “inserted a private part of his anatomy” into a dead pig’s mouth.

Downing Street sources said the story was "nonsense" since he was not a member of the society, but not before it sent Twitter wild and prompted politicians to weigh in with their own jokes.

The book was written by former Tory party donor Lord Michael Ashcroft and Isabel Oakeshott, a former Sunday Times political editor.

On the BBC's World At One, Ms Oakeshott defended the tale against the claim the story was "flimsy" since the sole source is an MP who also says there is a photograph of the incident.

The journalist said: "Look at the way it is written. If you look at the way it is written it is written extremely carefully, and I don't want to say anything more about it than that."

The extract in The Daily Mail reads:

A distinguished Oxford contemporary claims Cameron once took part in an outrageous initiation ceremony at a Piers Gaveston event, involving a dead pig. His extraordinary suggestion is that the future PM inserted a private part of his anatomy into the animal’s mouth.

The source — himself an MP — first made the allegation out of the blue at a business dinner in June 2014. Lowering his voice, he claimed to have seen photographic evidence of this disgusting ritual.

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.

Some months later, he repeated it a third time, providing a little more detail. The pig’s head, he claimed, had been resting on the lap of a Piers Gaveston society member while Cameron performed the act.

The MP also gave us the dimensions of the alleged photograph, and provided the name of the individual who he claims has it in his keeping.

The owner, however, has failed to respond to our approaches. Perhaps it is a case of mistaken identity. Yet it is an elaborate story for an otherwise credible figure to invent.

Ms Oakeshott goes on to tell the BBC: "This story has captured the public imagination, it's very funny, we used it, we ran it after some consideration because it's colourful and made us laugh. But it's a few paragraphs in a very long book."

Downing Street has been briefing journalists that Mr Cameron was not a member of the club, and that it is very different from the Bullingdon Club he was famously a member of.

But Ms Oakeshott said: "I am not expert on the Piers Gaveston club but from what I have gathered there is no membership of the Piers Gaveston club. It's a loose affiliation. It's not like the Bullingdon. If you Google the Piers Gaveston you don't see formal pictures of Piers Gaveston members."

Front page of Monday's Daily Mail

She added the book was published, as promised, after the general election and was "objective". "There is an awful lot of very positive things in the book," she said. "Even the conclusion itself is quite positive."

The journalist also dismissed the suggestion the book was fuelled by "revenge" from Lord Ashcroft over missing out on a top Government job after the 2010 election despite pumping millions of pounds into the Tory coffers.

She said: "If this was just a revenge job Lord Ashcroft and I could have published it before the election. It could have caused far more damage. We could have published the book over party conference. David Cameron has said he is not going to be serving a third term, so as party leader he is not going to go on and on. By publishing at this point it will do the least possible damage."

She added: "This is the best possible timing from his point of view. He is not going to be standing again."