Liberal Democrat leader Nick Clegg is preparing a last-ditch appeal for his MPs to avoid a split over tuition fee hikes.
Frantic negotiations have been continuing as some members of the Government, including transport minister Norman Baker, appeared willing to quit rather than back the controversial proposals.
Mr Clegg will hold a crunch meeting with his parliamentary party later at which he is expected to urge them to present a united front.
Lib Dem grandee Lord Ashdown has also appealed for MPs not to breach the coalition agreement by voting against the plans on Thursday, while Downing Street warned that ministers are still subject to the principle of collective responsibility.
However, there are signs that opposition within the party was hardening despite efforts to strike a deal.
Senior Tory David Davis will also vote against the rise in fees, his office has said.
The former shadow home secretary, who stood against David Cameron for the Conservative leadership in 2005, will rebel over the coalition's policy.