More than 100 Labour MPs are against the tuition policy
An education minister has said he would not be advocating government plans to allow universities to charge top-up fees if he thought the policy would damage the chances of "working-class kids".
Higher Education Minister Alan Johnson made the claim as the government faced a further rebellion from Labour MPs over the controversial proposals.
In a special debate on Monday, the Liberal Democrats, with Tory backing, have called for the plans to be abandoned.
Dozens of Labour MPs are known to disagree with the policy, which could see students charged up to £3,000 a year from September 2006.
Phil Willis, the Lib Dems education spokesman, urged Labour MPs to rebel against the plans, arguing they were based on a "nonsensical notion" that graduates earned up to £400,000 more over their careers.
He claimed top up fees would see student debt rise to around £21,000, which would put many people from poorer backgrounds off going to university.
This claim was backed by George Mudie, the Labour former lifelong learning minister, who asked Mr Johnson if he really believed "the nonsense" he was reading out.
But Mr Johnson said only graduates would pay top up fees "at a very advantageous rate" and stressed they would provide a "direct and predictable" source of revenue.
"I would not be in this post proposing this policy if I felt the proposals we are putting forward here would damage the ability of working class kids to go to university," he told MPs, adding: "This isn't a
pernicious, regressive regime."
But Mr Willis said 139 backbench MPs had signed an early day motion opposing top up fees, and two former secretaries of state for education, plus two ministers had gone on the record condemning the idea.
While recognising there was a crisis in higher education funding, he said top up fees were not the answer.
"What we are actually seeing is a government that is trying to get 50% of people into higher education and still persists in that nonsensical notion that all graduates suddenly become high earners," he said.
"Research demonstrates that students coming out of Oxbridge that have done arts degrees get no more in terms of their earning capacity than students who have left with two A-levels.
Willis: Urging Labour MPs to rebel against top up fees
"The reality is that things are different for different groups of students.
"David Beckham might not be a graduate, but he is one of the most highly-paid individuals.
"Yet without graduates who are able to mend his foot when it gets broken, look after his money, design his wife's clothes, deliver his children and complete his transfer to Real Madrid, he would be all the poorer."
More than 70 Labour MPs have expressed opposition to what is one of Tony Blair's flagship policies, but there is unlikely to be an open revolt, according to BBC political correspondent Guto Harri.
Legislation on the issue is not due until the autumn.
The government wants towards 50% of people under 30 in higher education by 2010
Annual tuition fees would rise to a maximum of £3,000, payable from graduate earnings
Families earning less than £30,000 would get help with first £1,100
From 2004, students with family incomes of less than £20,000 will get an annual grant on a sliding scale
Families earning under £10,000 will receive £1,000 a year
He said the policy would be lucky to survive, not only because of opposition to the principles, but because many Labour MPs regarded it as "politically suicidal".
A Department for Education and Skills (DfES) spokesman said putting off "difficult choices" would not help universities get the extra funding they need to thrive.
"If we don't address these tough issues and make choices, in the long term, the loser will be the British economy," he said.
"Our White Paper plans give universities the extra funding needed to thrive, whiles protecting students from unaffordable costs."
Meanwhile, shadow education secretary Damian Green called for tuition fees to be scrapped altogether.
"This would help students avoid the burden of long-term debt which deters increasing numbers of poorer students from applying to university," he said.