But the Conservative stance fell into confusion after the party's representative on the inquiry, veteran MP Michael Mates, suggested he might continue to serve.
Mr Howard's original decision to go along with the inquiry was questioned by some who warned that the remit given to Lord Butler was too narrow and that it would prevent him from getting at the truth of the real decisions which led to military action.
Some predicted that it would produce a "whitewash" - the same accusation that was levelled at the earlier Hutton inquiry.
Mr Howard, in a letter to Mr Blair, said that at the start he was satisfied that the terms of reference "provided the basis for a full and proper investigation." The letter goes on: "It has since become clear, however, that Lord Butler has chosen to interpret his terms of reference in what I regard as an unacceptable restricted fashion ... after careful reflection of these matters, I have, therefore, decided with regret to withdraw my cooperation."
