By Nick Assinder
Political correspondent, BBC News website
It is just possible that, more than three years after the Iraq invasion, Tony Blair will be defeated in the House of Commons over the war.
Mr Blair has rejected holding an inquiry while troops are in Iraq
The Tories have indicated they might vote with other opposition parties and Labour rebels to support demands for a full inquiry into all the circumstances surrounding the conflict.
The prime minister's spokesman has insisted to concede such an investigation now, or to defeat the government, would send out a message of weakness to terrorists and insurgents in Iraq and undermine British troops in the country.
But with the government holding a majority of 62 and 32 Labour MPs having already signed a Commons motion calling for such an inquiry, the arithmetic suggests the prime minister faces possible defeat.
The way these things work, a defeat would not mean Mr Blair had to resign, but an inquiry would then have to go ahead unless ministers managed to overturn the vote in the future.
However it would be a big blow to Mr Blair's authority in his last months as premier and as he continues to face pressure from many on his own side to quit Downing Street long before the May deadline he appears to have in mind.
Nudge or wink
Scottish National Party leader Alex Salmond, who is leading the Commons debate, has declared a defeat for Mr Blair would mean his tenure would be measured in hours and days rather than weeks and months.
Yet even Mr Salmond believes defeat is unlikely, and there are a number of reasons why he is almost certainly right.
The most powerful is that all the prime minister has actually signalled is that he is not prepared to make an announcement on any immediate or future, Falklands-style inquiry now.
Mr Salmond is leading demands for inquiry
But that does not rule out the possibility that, as the Tories are demanding, such an inquiry could be held once British troops have left Iraq and, likely, after Mr Blair has left Downing Street.
So any nudge or wink from the government that it is not entirely ruling out a future inquiry might be enough to buy off some rebels and Tory MPs.
And Mr Blair's official spokesman seemed to do just that in the hours before the vote, telling political journalists there was "no doubt at all that at the end of our period there (Iraq) people will want to look back. The time to deal with that is then, not now".
It is also the case that, at that time, Mr Blair will probably no longer be prime minister and the decision whether to hold a Falklands-style inquiry would fall to his successor who may, for his own reasons, be happy to see such an investigation.
Even without those hints it is also likely Tory leader David Cameron will face a minor revolt on his own benches with some of his most pro-war MPs refusing to vote with the anti-war nationalists on the issue.
Some have said the Conservative position smacks of opportunism from a party which supported the war. That, along with antipathy to the nationalists, will be enough to stop some Labour MPs rebelling.
But, even assuming the prime minister wins the day, that does not mean he will escape unscathed.
The debate will re-open the old arguments, wounds and divisions over the war and the way Britain was taken into it which have dogged the prime minister since before the invasion took place.
Worse, it comes at a time when doubts over the current position in Iraq appear to be increasing and when the outcome of the American mid-term elections could have a profound effect on US and, as a result UK, policy for the country.