DAVID Cameron has “a duty” to answer the serious allegation from Tory donor Lord Ashcroft that he misled the public for months over the peer’s “non-dom” tax exile status, Nicola Sturgeon has insisted.

The First Minister’s intervention will add to the pressure on the Prime Minister to make a response to the peer’s claims in his biography of the Conservative leader entitled Call Me Dave.

These also cover allegations of drug-taking and debauchery when Mr Cameron was an Oxford University student, including a lurid initiation ceremony, which was said to have involved him inserting "a private part of his anatomy" into the mouth of a dead pig. Friends of the PM have insisted the claims are “absolute nonsense”.

In the book, Lord Ashcroft says Mr Cameron knew in 2009 about his non-dom status, which allowed him to avoid tax on overseas earnings. This is denied by the PM, who insists he only became aware of it a month before the 2010 General Election.

Lord Ashcroft, whose millions of pounds in donations kept the Tory Party afloat following its landslide defeat in 1997, was made a peer in March 2000 by the then party leader William Hague and stressed at the time that he would become a UK resident for tax purposes. 

However, the ex-party Treasurer did not do so until years later when a change in the law meant he would have had to quit the House of Lords had he not become a UK tax resident.

Ms Sturgeon said of the claim over Lord Ashcroft’s former non-dom tax status: “This is an allegation among many allegations that David Cameron really has to address directly; it’s important that this allegation, that he knew more about Lord Ashcroft’s non-dom status than he had previously said he did, is not lost in the more lurid and humorous allegations that many people are talking about.

“This is a serious allegation and the Prime Minister has a duty to respond to it,” she added.

Labour’s Jonathan Ashworth, the Shadow Minister Without Portfolio, echoed the point, saying: “The Prime Minister should immediately clarify exactly when he first knew of Lord Ashcroft’s non-domiciled status.”

No 10, however, remained tight-lipped, declining to give any comment on anything contained in the book.

"I am not intending to dignify this book by offering any comment," declared the PM's spokeswoman. He (Lord Ashcroft) has set out his reasons for writing it. The Prime Minister is focused on getting on with the job of running the country."

Sources close to Mr Cameron said they "did not recognise" the “pretty low-rent” accusations, which include claims the PM was present at events where drugs were taken and was part of a decadent Oxford University dining society called the Piers Gaveston society; named after the lover of Edward II.

In Call Me Dave, due to be published next month, Lord Ashcroft acknowledges he has a personal "beef" with the PM after his failure to offer him a significant job in his administration following the formation of the Lib-Con Coalition in 2010.

He claimed Mr Cameron initially blamed his Liberal Democrat Coalition partners for blocking his appointment, before offering him a junior role at the Foreign Office, which the peer described as "declinable", adding: "It would have been better had Cameron offered me nothing at all."

But Nick Clegg made clear he had not blocked the peer’s appointment as a Minister. “No I don’t recollect that at all,” he declared, confirming, however, that he had blocked the appointment of ex-Tory leader Michael Howard as a European Commissioner. 

As the row over the book’s claims took off, Lord Ashcroft made clear they were “not about settling scores”. His co-author, journalist Isabel Oakeshott, also denied the motivation for the book was revenge.

“If this was just a revenge job, we could have published it before the election. That would have caused more damage,” said the former Sunday Times journalist, who argued that publishing extracts now would do the least amount of damage to Mr Cameron.