The government is facing a fresh revolt by backbenchers over variable university tuition fees.
Students face fees if the bill is passed into legislation
Leading Labour rebels have put down an amendment to stop universities introducing top-up fees.
They want to retain the rest of the legislation, which ends up-front fees and brings in a regulator to improve university access for poorer students.
But ministers have warned that they will withdraw the bill if the rebel amendment is passed.
Leading rebel Dr Ian Gibson said the amendment would cut out the bad things from the bill, leaving the good things.
Opponents to variable fees hope the move will encourage more Labour MPs to join the revolt on the third reading of the bill, which is expected next week.
They have written to all Labour backbenchers
asking for their support.
But the government is hoping the rebellion will be smaller this time around.
Under the plans universities can charge up to £3,000 a year from 2006, but opponents claim the fees will lead to a two-tier system where bright students from poor backgrounds are put off applying to the best universities because of high fees.
Seventy two Labour MPs voted against the government when the bill came before the Commons in January.
The Higher Education Bill was backed by 316 votes to 311, after days of intense campaigning by both sides.
Tony Blair had staked his authority on winning the vote, which saw a sharp reduction in his usual 161 majority and was widely seen as his biggest parliamentary test as prime minister.
His official spokesman said: "This is not a pick-and-mix
bill. It stands as a whole and variability is a key component.
We won the day in January and we will continue to work hard to ensure that position holds when the bill returns.
"There has been genuine dialogue but we believe this Bill is the best way forward for students and the higher education sector."
A spokesman for the Department for Education and Skills said the government's bill had already undergone "some important changes not least because of the ongoing dialogue between the government, stakeholders and backbench MPs".
Tory education spokesman Tim Yeo said the amendment was "further evidence" of deep divisions in Labour over the bill.
"The majority of MPs oppose top-up fees - the only way that the bill will pass is if government whips carry on using strong-arm tactics to force their MPs to
vote against their consciences."
For the Lib Dems Phil Willis said the bill was "bad for students, bad for universities, bad for taxpayers and bad for the future of our country".
"Next week's third reading gives Labour backbenchers a last chance to honour their manifesto pledges and save our universities from becoming a market lottery
for future students."