Tony Blair faces the risk of a humiliating Commons defeat today over his refusal to allow a wide-ranging inquiry into the crisis in Iraq. The Tories, Liberal Democrats and as many as 40 Labour rebels are threatening to support a nationalist demand for a parliamentary examination of the war and its aftermath.
The suggestion of any investigation is being fiercely opposed by Downing Street, which argues that it would undermine the British forces and give succour to Iraqi insurgents.
David Cameron turned up the pressure on the Prime Minister by warning that the Tories were ready to vote against the Government unless it bowed to demands for an inquiry.
He wants the Government to set up an investigation similar to the Franks inquiry that examined the background to the Falklands War in 1982.
The Tories had been expected to abstain in the Iraq debate - the first on the subject in the Commons for more than two years - which has been initiated by the Scottish Nationalist Party and Plaid Cymru. Their change of heart raises the prospect of Labour's working majority of 67 being wiped out if between 30 and 40 rebel MPs oppose the Government.
One Labour left-winger predicted that as many as 45 Labour MPs could join the opposition lobbies. He said: "We think this is going to be a very close vote. There has to be a full public inquiry into what went wrong and the lies that were told, both to the British public and to MPs."
Defeat for Mr Blair on the centrepiece of his foreign policy would be hugely embarrassing and could reignite dissent in Labour ranks over his leadership.
Today's SNP/Plaid Cymru motion calls for a select committee of seven senior MPs to investigate the Government's conduct in the run-up to the invasion of Iraq and during its aftermath.
The Tories want a wider investigation to be carried out by leading civil servants and soldiers in the next parliamentary session when British forces could be leaving Iraq. William Hague, the shadow Foreign Secretary, accused the Government of "dogmatically" opposing an investigation. He said: "A responsible government should want all possible lessons to be learned from the efforts to bring order and reconstruction to Iraq and should not be afraid of giving these issues the most searching examination."
But the Prime Minister's spokesman made clear it would not contemplate such an inquiry, although he did not rule it out once British soldiers had left Iraq.
He said: "We have troops who are operating in the field of combat. We have an enemy who is looking for any sign of weakness at all, any sign of a loss of resolution or determination. The important thing is that we do not give any signal that we are anything less than fully determined to see the job through."
Alex Salmond, leader of the SNP, said: "If the motion were to be carried, the Prime Minister's tenure in office would be measured in hours and days ... but even if the Government wins it by a narrow margin - which I think is the reality - then he would be in a Neville Chamberlain situation where you win the battle but lose the war.
"This is an opportunity for the House of Commons to bring to account a government which has led us into this bloody quagmire."
Michael Moore, foreign affairs spokesman for the Liberal Democrats, called on the Government to follow the lead of the White House, which has established the Baker review into the situation in Iraq. "This is an important debate which ought to be the starting point for government accountability on Iraq," he said. "It is unacceptable that the Government has not allotted time to debate this important issue for over two years and that we have had to rely on an opposition day debate before MPs can discuss this in Parliament."