Lord Ashcroft 'not settling scores' with David Cameron book
Ex-Conservative Party treasurer Lord Ashcroft says his new book about David Cameron is "not about settling scores".
The unauthorised biography, serialised in the Daily Mail, includes allegations about the PM's student days.
The former Tory donor admitted to having personal "beef" with the prime minister after not being offered a major job in the coalition government.
The prime minister's spokeswoman said she did "not see the need to dignify the book by offering any comment".
"The author has set out his reasons for writing it," she added.
In the book, Lord Ashcroft, who donated millions of pounds to help the Conservatives fight marginal seats, says Mr Cameron was aware of his non-domiciled tax status, which was heavily criticised by Labour, in 2009.
The book also includes allegations of drug-taking and an initiation ceremony Mr Cameron is said, by an unnamed source, to have taken part in, involving a dead pig.
But sources said Mr Cameron was never a member of the club in question, the Piers Gaveston Society, during his time at Oxford University, BBC assistant political editor Norman Smith said.
He added that the as yet unpublished book, Call Me Dave, was "less a story about stupid student pranks, and more a story about raw political revenge".
This was denied by journalist Isabel Oakeshott, who has co-written the book.
She told BBC Radio 4's The World at One: "If this was just a revenge job, Lord Ashcroft and I could have published it before the election."
Ms Oakeshott also said the Piers Gaveston Society did not have a formal membership list, saying "people go along" to society gatherings.
Analysis by Laura Kuenssberg, BBC political editor
Two powerful men need each other, and then one of them becomes more powerful, and doesn't need the other anymore.
The understanding which they had comes to break down, recriminations follow, and sometimes, full throttle revenge.
But the roots of the embarrassment that has sent social media into overdrive in the last day were laid before Twitter was invented.
In fact, the origins of the spectacular parting of ways are 15 years ago.
In the preface to his book, Lord Ashcroft says that after David Cameron became PM in 2010, he was offered the job of junior Foreign Office whip.
"After putting my neck on the line for nearly 10 years - both as party treasurer under William Hague and as deputy chairman - and after ploughing some £8m into the party, I regarded this as a declinable offer," he wrote.
"It would have been better had Cameron offered me nothing at all."
But he added: "Despite my disappointment, my new book about Cameron is not about settling scores."
Who is Lord Ashcroft?
- Ennobled by William Hague in 2000
- Has been deputy chairman and treasurer of the Conservative Party
- Donated millions of pounds to the party
- Gave up his non-domiciled tax status, which had been attacked by rival parties, in 2010
- Stood down from the House of Lords earlier this year
- Ranked 74 in the 2015 Sunday Times Rich List
According to Lord Ashcroft's account, Mr Cameron told him Lib Dem leader and former deputy prime minister Nick Clegg had vetoed his appointment to a government role.
Speaking on BBC Radio 4's The World at One, Mr Clegg said he did not recall being asked.
"I'm now used to Conservatives - they certainly did it for five years - using me as an alibi for awkward decisions that they had to face within their own party and I'm certain they fell into that category," he said.
Lord Ashcroft was deputy chairman during Mr Cameron's period as leader in opposition.
In July 2010, he gave up his non-domiciled tax status to remain in the Lords after a law was passed requiring peers and MPs to be tax resident and domiciled in order to remain in Parliament.
His tax status had long been criticised by his opponents.
Labour's shadow minister Jonathan Ashworth has written to Downing Street asking whether Mr Cameron was, as Lord Ashcroft claims, aware of his tax arrangements in 2009.
Scottish First Minister Nicola Sturgeon said Mr Cameron should "address directly" the claim, describing it as a "serious allegation".
In March Lord Ashcroft said he was stepping down from the House of Lords because he could not devote enough time to it.
In recent years he has built a reputation as an independent pollster.