Tony Blair is facing another Commons rebellion over plans to create dozens of "foundation hospitals".
What are foundation hospitals?
The proposals would see hospitals opting out of government control and becoming independent not-for-profit organisations.
They would be able to borrow money on the private markets and set their own financial and clinical priorities.
They would remain part of the NHS and be monitored by stakeholder councils, whose members would be drawn from local communities.
Who is objecting to the proposals?
About 100 Labour MPs have signed a Commons motion opposing the policy.
Some are regular critics of the government - but worryingly for Tony Blair, many are former ministers and MPs usually loyal to the prime minister.
What are their concerns over the policy?
The MPs say foundation hospitals would create "a two tier NHS system".
They argue that all NHS services should be brought up to the best possible standards.
It is for many an issue of principle. Equality should be the building block upon which improvements in the NHS are built, they say.
Essentially, they fear foundation hospitals would be able to offer staff higher wages and therefore attract the best workers.
As a result, they say, a divide would be created between foundation hospitals and other hospitals.
What are the arguments in favour of foundation hospitals?
Supporters believe that allowing some hospitals to raise private finance on the open markets will create centres of excellence and drag other hospitals up with them.
They say the idea would set hospitals free to respond to local need.
Mr Blair told the Labour Party conference last autumn that he wanted an end to the "one size fits all" mass production public service.
The prime minister believes the answer to the problem of delivering better health services is foundation hospitals.
So far more than 30 hospitals have applied to be given the freedom to raise money to boost their budgets, set pay levels and appoint the staff they want.
Why is the opposition to the plans serious for the government?
Any debate about the NHS is hugely sensitive for Labour.
Many MPs see the foundation hospitals proposals as eroding the principles of equality in the NHS.
And Labour leaders will be concerned that many of those objecting to the plans are not the "usual suspects" who have criticised earlier policy plans.
On top of the Iraq rebellion, a major revolt would raise further questions about the direction of New Labour and intensify the debate at the heart of the party about its future course.
Could the policy be rejected in the House of Commons?
The government would probably be able to get the plans on the statute book - but not without a major rebellion from Labour MPs.
Anything on the scale of the revolt over possible military action in Iraq would be a serious blow for the government.
Indeed, with the Tories backing the proposals, it is possible that ministers would have to rely on Conservative votes to get the policy through.
To have to do that on a major plank of domestic policy would be hugely embarrassing, not to mention damaging, for the government.
Some MPs believe the only way the government can avoid a big rebellion is to offer concessions on the plans.
Is the Cabinet united on the plans?
There have been tensions between the Treasury and the health department.
Gordon Brown is concerned about foundation hospitals borrowing on the private markets for major projects and then facing large debts.
In essence, he is worried about the Treasury being called upon to bail foundation hospitals out.
Under a deal brokered by Tony Blair between the chancellor and Health Secretary Alan Milburn, it was agreed that funding for major projects would have to be included in the NHS budget.
In other words, if foundation hospitals run into trouble, they would not be able to turn to the Treasury for extra cash.
If all sides stick to that agreement, Mr Brown's support can probably be relied upon. But there is little doubt that he is not the policy's most enthusiastic advocate.