David Cameron faced pressure over when he knew about Lord Ashcroft’s controversial tax arrangements as the Conservative peer made a succession of lurid claims about the Prime Minister’s past.

He alleged that as an Oxford student the future Prime Minister had taken part in a bizarre ritual in which he “put a private part of his anatomy” into a dead pig’s mouth and had belonged to a “dope-smoking group”.

In a new biography of Mr Cameron, Lord Ashcroft suggests that the Tory leader misled the public over the peer’s non-dom status ahead of the election in May 2010.

When details emerged in March 2010, senior Tories insisted that he had only just learned about the peer’s tax position and Mr Cameron said the full details were “only” known by Lord Ashcroft and the Inland Revenue.

But their version of events was contradicted by Lord Ashcroft who says: “In 2009, I discussed the matter in detail with Cameron. He was therefore fully aware of my status as a so-called ‘non-dom’.”

The Prime Minister’s official spokeswoman refused to be drawn on the peer’s claims. She said: “I’m not intending to dignify this book by offering any comment or any PM reaction to it.”

Asked when Mr Cameron knew about Lord Ashcroft’s non-dom status, she replied: “I’m not going to get into that; that predates the time of not just this government but the last government.”

Jonathan Ashworth, the shadow Minister Without Portfolio, said there was a “serious question mark over the consistency of the Prime Minister’s statements”.

He said: “The Prime Minister should immediately clarify exactly when he first knew of Lord Ashcroft’s non-domiciled status.”

The Scottish First Minister, Nicola Sturgeon, told Channel 4 News that the allegation “perhaps shouldn’t just be allowed to disappear into the ether with some of the more lewd ones”.

Lord Ashcroft’s book, which he has written jointly with the journalist Isabel Oakeshott, comes five years after he fell out spectacularly with the Prime Minister. As a major Tory donor and party treasurer he had expected a significant job in the Coalition government in 2010, but was only offered a junior post, which he rejected.

The peer’s most salacious claim is that Mr Cameron participated in an initiation ritual involving the pig for joining the Piers Gaveston club.

Lord Ashcroft said he was told about the incident by an Oxford contemporary of Mr Cameron who is now an MP and who claimed to have seen a photograph of the event. The claim prompted frenzied speculation over the MP’s identity. The club counts Mayor of London Boris Johnson among its former members.

Ashcroft: Looking back in anger

Lord Ashcroft’s revelations about David Cameron need to be read through the prism of the author’s motivations. While Ashcroft claims his book is “not about settling scores”, a reading of the first extracts, in the Daily Mail, suggest this should be taken with a pinch of salt.

Why else include highly inflammatory allegations about the Prime Minister and a pig when he has no hard evidence to prove it is correct? Why else spend thousands of pounds offering substantial payments to old friends, confidants and even disgruntled cabinet ministers to spill the beans on the Prime Minister?

When Cameron won the leadership in 2005, Ashcroft was the Conservatives’ biggest donor. Despite misgivings, Cameron needed Ashcroft’s money to keep the party afloat. But Cameron always thought Ashcroft was a liability to the party. And when he won the election in 2010 (in part thanks to Ashcroft money), Cameron unceremoniously dumped him.

Ashcroft believed he had been promised a significant role in the new government – but the call never came. In revenge Ashcroft stopped giving money to the Tories, set up his own political polling company (publishing data which previously only the Tories had had) and then embarked upon his book – delighting in the discomfort it caused Downing Street.