In a letter to Prime Minister Tony Blair, Tory leader Michael Howard said he could not accept the "unacceptably restrictive fashion" in which inquiry chief Lord Robin Butler was planning to carry out his work.

In particular, Howard said there was "no justification" for Butler to focus on "structures, systems and processes" and not the actions of individuals in the run-up to the US and British invasion of Iraq on March 20 last year.

"After careful reflection of these matters, I have, therefore, decided with regret to withdraw my cooperation from the Butler review," he said.

Blair's office reacted to Howard's letter by saying that the inquiry -- announced in early February as it became clear that Iraq no longer had weapons of mass destruction when the war began -- would go ahead.

"This is an independent inquiry looking independently at the issues and it has a broad-based membership," Blair's spokesman said.

"Whether others choose to be a part of it or not is a matter for them. But this inquiry will continue."

Despite withdrawing his party's support, Howard said the Tories' representative to the inquiry, Michael Mates, would be free to sit in on its deliberations "in his personal capacity".

Setting up the Butler inquiry marked an embarrassing U-turn for Blair, who had cited Iraqi leader Saddam Hussein's long-running pursuit of weapons of mass destruction as the prime reason for Britain joining the United States in invading Iraq.

Butler, a one-time aide to Margaret Thatcher, Conservative prime minister during the 1980s, now sits in the House of Lords, the upper house of parliament, after retiring as one of Britain's top civil servants.

The Conservatives backed the Iraq war. More recently, however, they have sought to reap political capital from the absence of any hard proof that Saddam still had weapons of mass destruction.

Britain's second opposition party, the Liberal Democrats, has already said it will not take part in the Butler inquiry because the probe will not consider the political dimension of Britain's entry into the war.

In January a judicial inquiry into the suicide of government weapons expert David Kelly cleared Blair and his inner circle of allegations that they had distorted the threat of Iraqi chemical, biological and nuclear weapons.

WAR.WIRE