Sir Menzies Campbell has said he is likely to vote against any proposal to raise university tuition fees.
The former Liberal Democrat leader said he had signed a pre-election pledge not to raise fees and would find it "very difficult" to go back on that.
Under the coalition agreement, Lib Dem MPs are permitted to abstain on the issue.
But Sir Menzies said he would be likely to go further and rebel against his party for the first time.
The issue of whether to lift the current £3,225 cap on student fees in England is being reviewed by an independent committee headed by former BP boss Lord Browne.
Many leading universities want to charge up to £6,000, saying this will help them support research and compete internationally but students and lecturers are opposed, saying it will restrict access to higher education to the wealthy.
The Liberal Democrats have long been committed to phasing out tuition fees but decided before the election this was unaffordable in the immediate future and could only be achieved over the medium to long term.
As part of their coalition deal with the Conservatives, Lib Dem MPs have been given leeway to abstain on a number of issues - such as tuition fees, nuclear power and tax breaks for married couples - where they fundamentally disagree with their partners' policy.
But Sir Menzies, who is chancellor of St Andrew's University, told the BBC's Daily Politics he had signed a pledge to oppose any rise in fees.
He said he wanted the position in England and Wales to be the same as in Scotland - where Scottish residents are not liable for fees.
"I am against this policy," he said.
"I have never voted against my party in the past but this is an issue of some importance and I signed a pledge, and I would find it very difficult not to reflect that pledge in my vote," he said.
"I would find it very difficult, impossible to vote for and very difficult, I think, on an issue of this kind simply to abstain."
Sir Menzies said it would be up to each Lib Dem MP individually to decide what to do if it came to a vote on the issue.
Students in England and Northern Ireland and non-Welsh residents at universities in Wales have to pay tuition fees of as much as £3,225 a year although this will rise to £3,290 from September.
Welsh residents studying in Wales pay fees of £1,285 while there are no tuition fees for Scottish students at institutions in Scotland.