The prime minister has mounted a full-scale offensive to sell his policy of top-up fees to hostile Labour MPs.

Amid unprecedented opposition Tony Blair addressed a meeting of the Parliamentary Labour Party where he issued a stout defence of his plans.

Offering no concessions he urged rebel MPs to focus on the big picture by setting the reform in the context of wider modernisation of public services.

But the pep talk is unlikely to quell trenchant opposition to proposals which could lead to the first policy defeat of his leadership.

Nearly 150 Labour MPs have signed a Commons early day motion protesting about the plans.

Defeat on a key plank of the government's reform agenda could prove fatal for the prime minister - who has already characterised the issue as one of confidence in his leadership.

In a bid to avoid an early defeat, senior ministers have agreed to delay the legislation until the New Year.

Despite the looming revolt, Blair stood firm on the issue when he was confronted in parliament.

"The government proposals, by abolishing the upfront fees altogether and making it a graduate repayment scheme, make the proposals far more fair than the existing system or certainly what went before," the prime minister said in the Commons.

"We need to widen access to our universities, we need to fund it, we need to fund it fairly and it is important that we manage to do that without putting all the burden on the general taxpayer, the vast majority of whom have not been to university."

But there are still strong suggestions that top-up fees could lead take Blair's leadership to the brink.

Over recent days pressure has grown on the chancellor, who has failed to give a ringing endorsement of the proposal, to back the plans.

In a speech on Tuesday night Gordon Brown gave his support in a carefully-worded section of a pre-arranged speech.

"It is right to move ahead with the reforms in the structure and funding of higher education," he said.

"It is right that once students become graduates they make a greater contribution.

"That is why as Tony Blair has said today it is essential that our reforms proceed through the House of Commons."

But privately Brown is said to have expressed strong concerns about the policy.

Other senior minister said to be opposed to top-up fees include John Prescott and Margaret Beckett.

Earlier this week chief whip Hilary Armstrong is said to have warned Blair that a vote on top-up fees could bring a series of resignations from ministerial ranks.

In a Commons debate on the government's plans, MPs were divided over the issue.

"We need to look at what the appropriate balance is between the contribution an individual makes and the contribution society as a whole makes," said Labour MP Richard Burden.

"There is an equation and a judgement we have to make about when a teacher is trained, how much is that the individual benefit to the teacher, and how much of it is to the benefit of society as a whole. The same with a doctor, the same with so many others.

"That's the discussion we should have, certainly with ourselves, certainly across the political world, but also with the people we represent."

But Lorna Fitzsimons, a former chairman of the National Union of Students, said she would support the policy.

"The biggest barrier to access for people that come from my community is the aspiration to go into higher education, not the issue of debt," she told MPs.

"When we had full grant only 20 per cent of people from the poorest communities ever went into British universities."