The committee probing intelligence on Iraq's weapons of mass destruction has vowed to continue its inquiry - despite both opposition parties boycotting it.
There have been accusations that the inquiry's remit is too limited
Conservative leader Michael Howard withdrew Tory support, saying inquiry chief Lord Butler's view of its remit was "unacceptably restrictive".
Labour is now the only party backing the inquiry after the Liberal Democrats refused to take part from the start.
Foreign Secretary Jack Straw accused Mr Howard of "shameless opportunism".
"Mr Howard agreed that the military action in Iraq was right, but lacks the backbone now to stand up for what he believed," he said.
"His approach carries no conviction and will impress no-one."
Senior Tory MP Michael Mates says he still intends to take his place on the inquiry team.
"I believe my duty is to continue to serve the review as best I can in the important tasks we have been given," he said.
He also joined committee members in insisting Mr Howard was wrong to suggest they would focus only on procedures for gathering and analysing intelligence, rather than considering the use made of it by politicians.
In a statement they said: "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."
Shadow foreign secretary Michael Ancram rejected charges of opportunism.
He said the way Lord Butler had interpreted his terms of reference did not reflect the prime minister's assurances that the inquiry would examine the way people used intelligence.
Instead, the focus was on "structures, systems and processes" and not people's judgements, Mr Ancram told BBC Radio 4's Today programme.
"That type of inquiry is not going to satisfy the doubts that exist in the public mind about the way the government handled intelligence in the run-up to the war," he argued.
Tony Blair called the inquiry after mounting pressure caused by the failure to find WMD stockpiles in Iraq, the US decision to hold an inquiry and remarks of former weapons inspector David Kay.
The prime minister said at the time the judgement on whether the intelligence available had justified military action was one for politicians, not the inquiry.
Liberal Democrat leader Charles Kennedy refused to be involved in the inquiry from the start because of that restriction.
Accusing the Tories of a dramatic u-turn, he told Today: "The position of the Conservative Party quite frankly is risible."
Mr Kennedy demanded the government publish the attorney-general's advice on the legality of the war.
And Mr Blair should make a statement to MPs on whether Britain was "complicit" in the alleged bugging of the offices of United Nations chief Kofi Annan, he added.