The shadow foreign secretary, William Hague, has said that there was no "secret deal" over the tax status of Tory benefactor Lord Ashcroft and that Tony Blair, who was prime minister at the time of negotiations about the peer's status, was informed of the details of the deal.
Documents leaked to the BBC have revealed further details about the negotiations which allowed the Conservative deputy chairman to become a peer even though he does not pay tax in the UK on his earnings overseas.
Mr Hague, who was Conservative leader at the time of Lord Ashcroft's agreement with the tax authorities, told Today presenter Evan Davis that he made a "mistake" by saying in a letter to Downing Street at the time that the decision to become a resident would cost Lord Ashcroft "tens of millions of pounds" a year in tax payments.
"The one thing I will concede on this - and which I think in retrospect was a mistake - was to say tens of millions because it may have cost him millions. We don't know, it may cost him millions into the future," he said.
"None of us can know - other than him, I suppose, and the tax authorities - what it has cost. But it was certainly an important change for him to go from being not resident in the UK."
And Mr Hague revealed that he became aware at the beginning of this year of Lord Ashcroft's tax status - before the present Conservative leader David Cameron was informed.