David Cameron has avoided calls to explain if he is "telling porkies" over billionaire peer Lord Ashcroft's tax status.

Call Me Dave by Lord Ashcroft and journalist Isabel Oakeshott alleges Mr Cameron was aware the peer had not given up his controversial "non dom" tax status when he joined the House of Lords earlier than was previously admitted.

Labour's Kevin Brennan also mocked the Prime Minister by joking about another allegation contained in the unauthorised biography, which claimed Mr Cameron participated in a stunt while at Oxford University involving a dead pig and "a private part of his anatomy".

Sources close to Mr Cameron have previously dismissed this claim as nonsense.

The PM replied by offering to lend his free copy of Lord Ashcroft's book to Mr Brennan in order to prevent anyone receiving royalties.

Speaking during Prime Minister's Questions, Cardiff West MP Mr Brennan asked Mr Cameron: "Can you help clear something up for the House and the country and it concerns the recent biography of you by Isabel Oakeshott?

"In it Lord Ashcroft says he told you in 2009 about his non dom tax status and yet in 2010 you said you didn't know the detail of Lord Ashcroft's tax status.

"Clearly someone is telling porkies. Is it you or Lord Ashcroft?"

Mr Cameron replied: "What I'd say to you is I would think of many better uses of your time than reading that book.

"I managed to procure a free copy and in order not to give anyone royalties I'll gladly lend you a copy if that's what you'd like.

"I think you'll remember in this House, Labour and Conservative agreed to legislate so that non doms could not sit in either House - legislation I fully supported and indeed suggested at the time."