Labour shadow minister Jonathan Ashworth has called on David Cameron to reveal when he knew about Lord Ashcroft's 'non-dom' tax status.
He said: "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."
Lord Ashcroft wrote that he made the Prime Minister fully aware of his non-domiciled status in 2009, and that the two had discussed keeping it secret until after the general election.
However in March 2010, when his status was made public, David Cameron said the full details were 'only' known by Lord Ashcroft and the Inland Revenue.
"The Prime Minister should immediately clarify exactly when he first knew of Lord Ashcroft's non-domiciled status," Ashworth added.