David Cameron has accused Labour of "rank hypocrisy" in opposing plans to raise tuition fees in England, accusing the party of lacking an alternative.
But leader Ed Miliband urged ministers to reconsider, saying they were "pulling the ladder" from the poor.
The coalition is expected to get its plans through, despite opposition from Labour and some Lib Dem and Tory MPs.
But Energy Secretary Chris Huhne will miss the vote as he will remain at the climate summit in Mexico.
Lib Dem officials confirmed he would remain at the climate change conference in Cancun - it had been thought he had been called back for Thursday's vote.
Another Lib Dem MP, Martin Horwood, who was expected to vote against the fees package, will also remain in Mexico but Conservative climate minister Greg Barker will return to Westminster.
Liberal Democrat deputy leader Simon Hughes told BBC Newsnight he would at least abstain on the vote - but said he had been asked by his local party to consider voting against the plans and he would "reflect" on that request overnight.
More than a dozen Lib Dems are expected to rebel, by voting against the plans to raise the tuition fees ceiling from £3,290 to £9,000 a year. They say they have no choice as they signed a National Union of Students (NUS) pledge to oppose any increase.
At least two Conservatives are also expected to defy the government.
On Wednesday evening a coalition source accused the NUS of having proposed to "drastically cut support for low income students and decimate our universities" in a series of emails in which the union suggested cutting maintenance grants, teaching support and research funding - in order to maintain fees at their current level.
NUS President Aaron Porter accused Business Secretary Vince Cable of having deliberately misled Parliament over the NUS stance on student numbers - and said they had produced "modelling" to show how fees could be kept at current levels at his request.
During heated prime minister's question time exchanges, Labour leader Mr Miliband said the coalition was slashing public funding for universities and "loading the cost onto students and their families".
"The most sensible thing is to go away, think again and come up with a better proposal," he urged.
But Mr Cameron said Labour had no alternative scheme and said that the party's "rank hypocrisy" would harm the chances of poorer students getting to university.
In a speech to the Centre Forum liberal think-tank in central London, the prime minister said the proposed funding changes would allow the best universities to become "even better".
Raising the earnings threshold at which graduates pay back fees from £15,000 to £21,000 would mean "only the successful" would be affected, he added.
Ministers offered concessions designed to win over wavering Lib Dem backbenchers - the party's ministers will all vote in favour of the changes.
The government announced the salary threshold at which graduates start to repay fees will be uprated each year in line with earnings from 2016 - not just every five years, as had been planned.
Other concessions included uprating the existing £15,000 repayment level by inflation from 2012 and enabling part-time students to apply for student loans if they study for a quarter of the year, rather than a third as planned.
Labour said it would back a cross-party amendment to the tuition fees motion, which calls for the decision to be delayed until after the government has consulted more widely and published its higher education White Paper.
Shadow business secretary John Denham said proposals were being "rushed through without the consequences being thought out" and called on MPs to back the amendment. "If the amendment falls we hope that MPs from all parties will join us in blocking the legislation," he said.
Former Lib Dem leaders Charles Kennedy and Sir Menzies Campbell are among MPs set to oppose the plans, which have prompted weeks of student protests. Others are expected to abstain.
Lib Dem leader and Deputy Prime Minister Nick Clegg told the BBC that some ministers would have "doubts, reluctance and anguish" but understood the plan represented the "best and fairest possible deal".
While the policy of the government is to support a rise, under the terms of the coalition agreement Lib Dem MPs, including ministers, are allowed to abstain on the issue.
However, Mr Clegg said any Lib Dem parliamentary private secretaries - MPs who provide support and advice to ministers - who voted against the plans would find it "difficult to carry on" in the unpaid role.
Lib Dem MPs signed a pledge organised by the NUS during the election campaign to oppose any future rises in fees.
The party's manifesto also promised to phase out tuition fees altogether within six years. The leadership says that forming the coalition has meant compromise on this policy is necessary.
But the NUS has threatened to campaign directly against those who break their promises.
Meanwhile, shadow chancellor Alan Johnson - who has previously opposed a graduate tax - told the Times he now believed there was a "strong case" for one. He accused the coalition of "abusing the legacy I left them".
But Mr Cameron said: "Anyone with a brain who has looked at it realises it [a graduate tax] doesn't work."
How do you think Liberal Democrat MPs should vote? How should universities be funded? Are you a parent supporting or opposing the current proposals?
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