The Conservative Party has pulled out of the Butler inquiry into intelligence on Iraq's weapons of mass destruction.
There have been accusations that the inquiry's remit is too limited
Michael Howard said Lord Butler's interpretation of the inquiry's terms of reference were "unacceptably restrictive".
But the Tory leader was accused of "shameless opportunism" by Jack Straw.
The foreign secretary said: "Mr Howard chooses to jump on a passing bandwagon and to flirt with those opposed to the war to win cheap political points."
A statement issued by the Butler committee expressed "regret" at Mr Howard's decision.
It continued: "When the full committee met Mr Howard on 17 February, the members made it very clear that they would follow the analysis wherever it led, including uncovering any faults by individuals.
"The committee must start by looking at structures, processes and systems before considering which, if any, individuals should be held accountable."
Earlier Mr Howard had said the inquiry should give equal weight to the actions of individuals and not just structures.
His argument was not with the remit agreed for the inquiry but with Lord Butler's interpretation of it.
The Lib Dems have already declined to take part in the review because they thought its guidelines too narrow.
Mr Blair called the inquiry after mounting pressure caused by the failure to find any WMD stockpiles in Iraq, the American decision to hold an inquiry and the remarks of former weapons inspector David Kay.
At the time the Tories believed its remit covered the way intelligence was used before the war, saying that meant politicians' actions would be investigated.
But the inquiry committee later made clear it would concentrate "principally on structures, systems and processes rather than on the actions of individuals".
Mr Blair said at the time that the judgement on whether military action was justified by the intelligence was one for politicians rather than the inquiry.
In a letter to the prime minister on Monday, Mr Howard said: "It has since become clear that Lord Butler has chosen to interpret his terms of reference in what I regard as an unacceptably restrictive fashion."
The Tory leader said he had been "satisfied" with the inquiry's terms of reference because he believed they would provide "a basis for a full and proper investigations into all aspects of the gathering, evaluation and use by the government of intelligence on Iraqi WMD up to March, 2003".
However, he discovered in mid-February that the probe would "not include amongst its aims an examination of the acts or omissions of individuals".
"It makes clear that it will consider such acts or omissions only in the context of its examinations of structures, systems and processes," said Mr Howard.
"There is no basis in the terms of reference for that view and I consider it a quite unjustifiable restriction on the committee's approach.
"After careful reflection of these matters, I have therefore, decided with regret to withdraw my cooperation from the Butler Review."
Mr Howard told the BBC he had "leant over backwards" to try to be helpful and agree the terms of reference.
The prime minister's official spokesman insisted that the inquiry's credibility depended on its "independent nature", when asked if it could retain credibility without the support of both main opposition parties.
Liberal Democrat leader Charles Kennedy said Mr Howard's change of heart had made his party look "completely opportunistic and quite frankly, ridiculous".
"The Butler inquiry will not be able to ask the fundamental political questions that people want answered in this country about the judgements that were taken .. by the people at the top in deciding to support President Bush in this war on Iraq," he said.
Senior Tory MP Michael Mates said he was still intending to take his place on the inquiry team.
"I appreciate that Michael Howard has made his decision on behalf of the official opposition, but I believe that my duty is to continue to serve the review as best I can in the important tasks we have been given."