Labour has forced a debate in the House of Commons over the abolition of the education maintenance allowance (EMA), as Simon Hughes, the Government's advocate for access to further and higher education and Lib Dem deputy leader, said his vote hung in the balance.
Mr Hughes faced booing from Save EMA campaigners at a parliamentary meeting after saying he was likely to abstain in the vote next Wednesday, when Labour dedicates an opposition day debate to the issue.
But at the meeting on Tuesday he accepted an offer by shadow education secretary Andy Burnham to try to negotiate wording for the motion that he and other Liberal Democrats could support.
The pressure on Mr Hughes demonstrates the cracks appearing in the Coalition over the future of education spending.
Mr Hughes became the Government's access advocate last month, a role that includes advising on the replacement for the EMA, which will cease at the end of this academic year. He told campaigners: "We haven't seen a motion yet. If I am meant to be giving advice on this to ministers, I will probably abstain." Campaigners booed and shouted: "Shame!"
Pressured by them further, he said: "Replacing a system where you know what you're getting with something where you don't know what you're getting isn't incentivising.
"Where's the logic in taking money and support away just at the time when you're expecting more people to stay on? I'm waiting for that answer.
"I've never abstained in my life before the tuition fees debate. If what Labour is saying is a call for the Government to rethink its plans, I will support that. There's some careful brokering to do."
Mr Hughes said his main concerns were the cost of travel, equipment and meals for students, and he added it was unfair for students who started two-year courses in September to be cut off halfway through.
But he warned it would not be possible to maintain the EMA at its full value of pound;564 million a year for 600,000 teenagers.
"I can't undo the decisions that were made before I was appointed on 28 December. The Government doesn't want the system to go on as it is. The money isn't there to the same extent. Whether we can square that circle, who knows? I can't deliver the impossible," he said.
Mr Burnham criticised the betrayal of promises made by prime minister David Cameron and education secretary Michael Gove not to scrap the EMA, and its abolition through an administrative order rather than a vote.
One student from Cornwall College told the meeting she had been personally promised by Mr Cameron that the EMA was safe when he visited the college during the election campaign last year. "What does that do to young people's trust in politics?" Mr Burnham asked.
Campaigners, the National Union of Students and supporters in the teaching unions intend to hold protests the day before the vote and to lobby MPs next Wednesday.