By Nick Assinder BBC News Online political correspondent

Tony Blair knew he faced an uphill struggle when he set himself the task of winning around opinion in favour of military action against Iraq.

And, this time, it appears the great persuader's charms have failed - even with his own ministers.

Short leads critics

Not only are the majority of the British people against war, a substantial number of his backbenchers and even his cabinet - possibly the majority - are opposed to unilateral action.

Those concerns will dominate this week's political agenda and the prime minister is under massive pressure to clarify his position.

He will face questions over his stand during his first televised press conference of the year this afternoon.

And on Wednesday he will be in the Commons for question time.

No answers

While Tory leader Iain Duncan Smith broadly backs his stand, there are many opposition MPs opposed to unilateral action, and support from that quarter is far from guaranteed.

The same day, the prime minister will also face his own backbenchers at a meeting of the Parliamentary Labour party.

Of course, if evidence is found, then it is quite possible the UN will be persuaded to back military action

He already knows what message he will hear at all these events.

The key questions he faces are whether he is prepared to back President Bush in a war against Saddam Hussein without UN backing or if the inspectors continue to find no weapons of mass destruction.

They are questions that have been posed on many previous occasions but have not been answered.

But, with the military build-up continuing, they are now being asked with new urgency.

On the question of UN backing, the prime minister has repeated his mantra that the body must be a way of dealing with the issue of Saddam, not avoiding it.

To most people that sounds like a slippery way of saying the UN had better come up with the right answer, and back military action, or Tony and George will go it alone.

Stand down

On the question of the inspectors' findings, the prime minister has also previously insisted that there is already evidence of Saddam's development of weapons of mass destruction.

That is what the government's much-criticised dossier was supposed to show last year.

Of course, if evidence is found, then it is quite possible the UN will be persuaded to back military action.

Troop build-up

It it also just possible that Saddam can be persuaded to stand down by some of his Arab friends.

Either of those outcomes would take the pressure off the prime minister.

But for now, Mr Blair will be asked these crunch questions again this week. Once again, however, he is expected not to answer them.

That will serve only to heighten fears and suspicions over the future.

Clare Short's comments, combined with the latest opinion polls, have underlined the seriousness of the situation facing the prime minister.

Leadership threat?

If he goes to war without UN backing and/or no concrete evidence of Saddam's weapons of mass destruction he will not only face a public backlash in Britain, he will be isolated in the global community and probably spark a major cabinet split.

It would be hard for Ms Short to remain in the cabinet if he took that route and, if she resigned, others might feel obliged to follow suit.

Since 12 September 2001 the prime minister's body language has been pretty clear on this

Many ordinary Labour party members may also quit the party and it is almost certain the government's popularity would slump.

It is not an exaggeration to suggest that such a course of action could even see a challenge to the prime minister's leadership.

Foreign Secretary Jack Straw has been careful not to be seen backing the prime minister's hard line.

And Chancellor Gordon Brown has done one of his traditional disappearing tricks.

So it is no surprise that the government currently appears to be talking with two different voices on the affair.

Doing the right thing?

Last week, much store was placed on the prime minister's insistence that the weapons inspectors must be given time to do their job.

That was seen as a softening of the line. In fact it has been the constant message from Downing Street ever since the UN agreed the strategy.

But it suited the prime minister's purposes to stress it at this particular time to suggest he is not as gung ho as many fear.

But from day one - that is from 12 September 2001 - the prime minister's body language has been pretty clear on this.

He genuinely believes Saddam is a real threat and that he is doing the right thing by preparing to finish him off.

So far, nothing appears to have changed that belief.