By Nick Assinder BBC News Online political correspondent

Blair may lose his MPs' votes

He is about to go to war with Saddam Hussein under the worst of all possible conditions.

After deploying all his powers of persuasion and expressing his confidence he would win the day both at home and in the world, he has failed.

There is no second UN resolution and he has started losing ministers.

So the prime minister now faces the greatest backbench rebellion not only of his premiership, but probably in recent political history.

There will be a Commons vote on the issue on Tuesday where his rebels will express their dismay that military action is being launched without a further UN resolution.

Tory support

And the prime minister now faces the real prospect that he will lose the majority on his own backbenches.

If 165 of his own MPs refuse to back him, and once the so-called payroll vote is taken out of the equation, he will be relying on the Tories to give him Parliamentary support for his action.

He saw 122 defy him in the last vote and the rebels are claiming as many as 40 or 50 others will join them this time. That is dangerously close to a majority.

Presumably he simply wants to get this over and done with and get on with the task in hand.

And that task will be undertaken even if he loses the support of his own MPs.

But no matter how the arithmetic finally adds up, or even when the vote comes, no one will be under any doubt that the prime minister will be acting against the wishes of a huge section, probably a majority, of his own party.

It is hard to overstate just how far that will inflame the backbench anger at his actions.

Political truce

There has always been a hard core of Labour MPs deeply opposed to his leadership. But this revolt goes far wider than the usual suspects.

And many of those rebels are as personally committed to their positions as the prime minister is to his.

They are likely to rally around Commons leader Robin Cook now he has resigned from the cabinet.

Meanwhile, the Tories will not publicly crow about it, but make no mistake, once the war is over they will privately use the fact Tony Blair relied on them as a stick with which to beat him.

Set for war without the UN

Also, once the temporary political truce imposed by the war is over, there will almost certainly be attempts to force a leadership challenge.

Even if the prime minister is proved right in his claims about Saddam's weapons of mass destruction and terrorist links, some will still want a challenge as a result of his actions.

So it is not impossible that the prime minister emerges from the war vindicated, but still under threat from his own MPs.

Under those circumstances the leadership challenges would almost certainly come to nothing.

But he will have become dangerously detached from his party.

If the war goes badly - and there are always a thousand ways such action can go wrong - he may be one of the casualties.