More than 150 MPs are against the fee policy

Mr Howard told BBC Radio 4's Today programme the plans were "absolute nonsense", and said more students should do vocational courses instead.

But Labour ministers have been rallying around the prime minister on the issue.

Chancellor Gordon Brown told the Sun he backed a system of variable fees repaid on graduation rather than paid upfront.

I am not opposed to it David Blunkett Home Secretary

Home Secretary David Blunkett denied leading a cabinet revolt on the issue.

And higher education minister Alan Johnson said the scheme would increase access for students from poorer backgrounds to the top universities.

He told Radio 4's PM programme it could close what he called the "obscene social class gap" that he said had dogged Britain's higher education system for 50 years.

Mr Blair is pressing ahead with his plans - which will allow universities to charge variable amounts for tuition, payable by students once they graduate and get a job - despite opposition from many Labour MPs.

He argues the measure is essential if the government is to achieve its target of getting 50% of young people into university.

But Mr Howard told BBC Radio 4's Today programme university drop-out rates and "Mickey Mouse courses" made the 50% target "difficult to justify".

Many people would benefit more from vocational training, he said.

'Significant debts'

It was also unfair to burden the "many decent hard-working graduates on modest incomes" with "very large debts they will have to repay until they are in their forties," he said.

He said the top-up fee scheme would not even cover the costs of the 50% target, anyway.

However you look at the government's proposal it is an absolute nonsense Michael Howard

"However you look at the government's proposal it is an absolute nonsense."

He later told BBC News 24 he agreed universities needed more money, but suggested other avenues such as "more finance from the private sector".

More than 150 Labour MPs have signed a motion against the variable fees plan.

The chancellor said he was confident the bill would make it onto the statute books, but admitted the public and MPs had to be persuaded "about the detail".

'Right policy'

Mr Brown said he was in favour of reforming the structure of university funding.

It is right to move forward with a variable fee option Chancellor Gordon Brown

"It is right that we should devise a system where, instead of an up-front payment, you have a retrospective payment.

"It is right to move forward with a variable fee option. And it is right that we should have a fairer system of repayment," he said.

Mr Blunkett, a former education minister, told Radio 4's Any Questions programme he was "not opposed" to the scheme.

"It is absolutely clear that we need to raise in the next parliament very substantial sums of money for the universities," he said.

"If we didn't raise it in this way we'd have to displace investment in other critical parts of public service, not least other parts of education which desperately need it."

'Fairest way'

Mr Blair's official spokesman said he believed the argument was being won.

We acknowledge this is a difficult issue Prime minister's official spokesman

"While it is a difficult issue that involves tough choices, the public understands the need for greater investment in universities," he said.

Mr Blair could face his first Commons defeat since he came to power when the issue is debated by MPs next month.

There was still time to persuade Labour rebels before the vote, Mr Blair's spokesman said.

"This is a debate that can run until the end of January. We acknowledge this is a difficult issue," he said.