By Nick Assinder BBC News Online political correspondent

Duncan Smith could take advantage of Labour revolt

With the government facing a major rebellion from its own MPs on Wednesaday over plans for foundation hospitals, the Tory leader knows he could actually help kill this policy off.

And, in doing so, he could probably take Health Secretary Alan Milburn's head - and possibly even go for the big one, the prime minister himself.

A defeat over this central policy would have seismic consequences for the Labour Party, the prime minister, and his leadership rival - and opponent of the reform - Gordon Brown.

There would be talk of votes of confidence in the prime minister and even leadership challenges. Mr Milburn has already used the quit word in the same breath as foundation hospitals.

And Mr Duncan Smith's leadership would be given the sort of boost he can only dream of at the moment.

This would put the cap on his local election success and transform him into the Tories' greatest living hero.

All that alone is enough to suggest it can't possibly happen and few in Westminster are foolhardy enough to take bets on it.

Damp squib

The post-war Blair is a far more determined and self-confident man that at any time during his premiership and he has already shown an impressive ability to see off major rebellions.

The usual pattern for such revolts is that there is much over-excited talk of approaching Armageddon beforehand, followed by some serious whipping and an eventual damp squib.

Deep concerns over foundation hospitals

But, as the historic revolt over the war proved, there is no law which states a prime minister with an unassailable majority will remain unassailed.

So, what is required for the government to be defeated over this issue?

First off it needs the Tories to take the political decision not to support the proposal in a Commons vote. Mr Duncan Smith has already announced the government cannot bank on its support.

The Liberal Democrats have already taken that decision.

Charles Kennedy is against the policy in principle and will lead his troops through the "no" lobby.

The Tories, meanwhile, may like the idea of foundation hospitals - some would even argue they more or less invented it in the first place - but that is no reason to support this Bill.

It is not beyond the wit of Tory bosses to find a credible argument for opposing it - as if the thought of scaring the smile off Tony Blair's face wouldn't be enough on its own.

Some Tories might believe that seeing a prime minister forced to rely on their votes to push through a piece of legislation against his own party's wishes would be sweet enough. But why go for second best.

Battle-hardened

The next thing that needs to happen is that all those Labour MPs who have signed the motion opposing the policy actually vote against their government - and a few more join in besides.

It may just be that the lingering anger over the way the prime minister took them into war will stiffen a few rebellious spines. But don't bet on it.

It is also just as likely that timid rebels will be frightened off by the new, steely-eyed, battle-hardened Blair.

Others will be bought off by concessions on NHS funding.

The prime minister is quite ready, even eager, to see these people off and is devoting a substantial part of his daily diary to meeting them and either piling on the pressure, or turning on the charm according to taste.

If any one of this doesn't play then the prime minister will once again be off the hook.

But what just about everybody in Westminster agrees on is that this is probably the biggest test now facing the prime minister.

He has shown he is ready and eager for a fight, it remains to be seen whether his opponents are.