The British Labour Party has said there is a "serious question mark" about David Cameron's statements on Lord Ashcroft's tax status.

The Opposition called on the British Prime Minister to clarify when he found out about the peer's non-dom tax status.

Lord Ashcroft says in a new book that Mr Cameron knew in 2009 that he had not given up his tax privileges, while the Prime Minister has said he did not know until 2010.

Jonathan Ashworth MP, Labour's shadow minister without portfolio, said in a statement: "Amidst the furore around Lord Ashcroft's new book there lies a serious question mark over the consistency of the Prime Minister's statements about the peer's tax status."

He added: "The Prime Minister should immediately clarify exactly when he first knew of Lord Ashcroft's non-domiciled status."

Lord Ashcroft gave up his non-dom status in 2010 so he could remain in the House of Lords. 

The peer, a former Conservative party treasurer and donor, has written a biography, Call Me Dave, that includes tales of excesses during the Prime Minister's university days.

Downing Street has said it refuses to "dignify" the book.

Mr Cameron made his first public appearance since the allegations emerged as he welcomed the Danish prime minister to Downing Street, but he did not speak to the media.

The book, which is being serialised in the Daily Mail, was co-authored by former Sunday Times political editor Isabel Oakeshott.

The Daily Mail described the book as Lord Ashcroft's "revenge" for being denied a job in government after the 2010 election, but Ms Oakeshott has denied this.

"Lord Ashcroft was always very clear right from the outset of this project that the book should be objective and even-handed," she told Sky News. 

Asked if Lord Ashcroft claims Mr Cameron lied over the non-dom status, Ms Oakeshott said: "Lord Ashcroft is very clear about the sequence of events and his recollection is what it is, he made notes at the time of the conversations that they had.

"We don't use the word liar but I think disingenuous would be a fair representation."

Ms Oakeshott said the state of Lord Ashcroft's tax affairs is a "serious political story" while salacious allegations about Mr Cameron's past are a "small anecdote - albeit one that has caught people's imagination".

She says that Mr Cameron is "absolutely fit" to be Prime Minister.