By Nick Assinder BBC News Online political correspondent

St Thomas's Hospital in London

So the health secretary will be less than delighted, if not surprised, that his plans for foundation hospitals are threatening the next big backbench rebellion.

He is an ambitious minister who has nailed his flag firmly to the Blairite mast - but that is starting to look a far less secure boat to be in than was once the case.

Still, the smooth operator has shown no sign that he is anything other than absolutely committed to the radical policy for the NHS, even though it is widely viewed by backbenchers as being Blair's baby.

And, as with the recent anti-war revolt, it is not just the usual suspects lining up to hammer the government over this policy.

Two tier

The Iraq rebellion has certainly stiffened the spine of would-be rebels and those opposed to foundation hospitals believe that will have the effect of increasing support for their cause.

Their criticisms of the proposals are widely shared in the union movement which is already on collision course with Tony Blair over his determination to bring private finance into the public services.

What the opponents fear is that the proposal will create a two tier health service with foundation hospitals sucking all the talent and resources away from the NHS.

Government reassurances that this is not the start of privatising the health service are treated with a huge chunk of scepticism.

After all, it is argued, if these hospitals are supposed to be "centres of excellence", as Mr Milburn insists, what does that make the rest of the NHS?

Tony Blair's insistence that he is to do away with a "one size fits all " NHS has also raised suspicions.

Opponents hear an echo of Alastair Campbell's infamous comment about "bog standard comprehensive" schools in this attitude.

Dr Ian Gibson, chairman of the Commons science and technology committee, summed up the rebels' feeling, declaring: "I think there is unease and disquiet, not just on this issue of the health service but on the general issue - in higher education too, where we see certain universities being selected out of the pack as the champions for Britain Plc."

Lost battle

They believe attempts to offload costs onto the private sector have not proved an unqualified success in other areas.

They also believe the policy is more about sucking up to the middle classes than improving the health service.

The policy has already been finessed after a full-scale bust up between the prime minister and Chancellor Gordon Brown.

But the bottom line was that the Chancellor lost the battle.

Nowadays, however, that only adds to the likelihood of a rebellion with MPs looking beyond Tony Blair to the next Labour leader.

It now seems that this issue may become the key battleground over Mr Blair's attempts to reform the public services.

It has been the centre of the government's programme since the last general election.

The prime minister promised more cash for the public services and has delivered on that pledge. Now he is calling in his markers on reform.