Tony Blair is only a few votes away from seeing a big piece of government legislation thrown out by Parliament this week, putting a brake on his plans to overhaul the NHS.
The proposal to allow the best-run NHS trusts to take a new status as foundation hospitals, with greater control over their own finances, will be the last piece of legislation to go through the Commons as the parliamentary year comes to a close.
MPs have already passed the measure once, but by a majority of only 35, the smallest for any piece of government legislation during Mr Blair's six years as Prime Minister.
This time, rebels are predicting that the Government's majority could be even smaller, increasing the likelihood that the Lords will block the legislation again.
A close vote in the Commons would mean a chaotic end to the parliamentary year, because after MPs have voted on the measure on Wednesday, it will have to go back to the House of Lords, where it was roundly defeated last time, by 150-100, with a substantial number of Labour peers voting against the Government.
If neither side gives way, the Government may have to keep the Commons sitting beyond Thursday, the day when the parliamentary year is scheduled to end.
Resistance to the measure has been stiffened over the weekend by the intervention of the main health unions and two influential Labour MPs.
Frank Dobson, the former health secretary, and David Hinchliffe, chairman of the Commons Health Committee, have written to their fellow Labour MPs warning that, despite government reassurances, foundation hospitals will be able to "poach" staff from other NHS hospitals by offering better pay packages.
They also forecast that MPs will face endless political problems locally if their nearest hospital is denied foundation status, and a different, but almost equally difficult, set of problems if it takes on foundation status.
Mr Dobson said yesterday he had been surprised by his fellow MPs' continued willingness to oppose the measure. It had been thought that the combination of Mr Blair's pugnacious speech to the Labour Party conference and the election of Michael Howard to the Conservative leadership would deter the rebels.
"We have yet to identify anybody who was previously opposed to foundation hospitals and who has changed their mind," Mr Dobson said, "but we have identified about a dozen who voted with the Government and who will now abstain, or who abstained and will now vote against."
MPs are also being lobbied by the leaders of the three biggest public sector unions, Unison, the TGWU and the GMB. Dave Prentis, head of Unison, which has 450,000 members in the NHS, wrote to Labour MPs yesterday warning that foundation hospitals could be "deeply damaging" to the NHS as well as to the Labour Party.
Mr Prentis said: "I believe that, if introduced, foundation trusts will lead to increased inequality and competition, undermining our ability to deliver our commitments on the NHS and generating dissatisfaction among patients and the public."
Unison said opinion polls had shown that a majority of the public was against foundation hospitals.
James Johnson, chairman of the British Medical Association, wrote to the Health Secretary, John Reid, on Friday to underline doctors' opposition to the proposals.