Ceredigion MP Mark Williams says it would be "dishonest" if he voted with the coalition government to scrap the upper limit on university tuition fees in England.
Lord Browne's review on university funding is set to recommend fees rise to £7,000.
Lib Dem MP Mr Williams said it was "huge principle" and he would vote against it.
There are also fears universities in Wales could lose out on funding.
Many Lib Dem MPs oppose the policy, which could lead to difficulties for the coalition in terms of a vote in the Commons.
The UK Government is also poised to abolish the zero interest rate on student loans.
Mr Williams says he will vote against the cap so as to stay true to his election manifesto, arguing a mere abstention is not sufficient to register his disapproval of the plans.
He told BBC Wales: "This is an issue of huge principle to Liberal Democrats.
"I've fought four elections in the Ceredigion constituency with two excellent universities - in Lampeter and Aberystwyth - and I've made commitments over those four elections and specifically at the last general election and on that basis I'm sticking by that pledge.
"I think it would be dishonest for me personally to do anything other than that and an abstention is not a vote against those increases."
He said the coalition government would need to listen to its backbenchers on this issue.
The coalition agreement drawn up back in May did anticipate discontent as it stated Liberal Democrat MPs will be able to abstain from the debate on Lord Browne's conclusions.
Mr Williams added: "I support the coalition document in its entirety but don't think an abstention vote is a sufficient way of measuring my discontent at what may be put forward.
"If what we've read in papers this weekend and heard on the TV and radio is the substance of what the government is putting forward, for me personally and some of my colleagues as well, that is unacceptable."
The issue is already presenting deep political challenges to the Liberal Democrats.
The party's MPs, including leader Nick Clegg, signed personal pledges at the election to vote against any increase in tuition fees - and any attempt to reverse this will bring them under pressure from students and their own grassroots supporters.
Alex Barker, political correspondent for the Financial Times, said the proposals would make grim reading for Liberal Democrats.
He said: "The problem for the Lib Dems is there aren't that many alternatives.
"They have to cut the subsidy to universities, there's no money left in the public finances, universities want more money in total as they say they are under-funded and we are running a student loans system at the moment where 24p in every pound is subsidised by the tax-payer.
"This is certainly the third rail - I don't think it will break the coalition at the top level between Clegg and Cameron but what I think it may do is inflict their first big defeat in the Commons.
"Not that many Lib Dem MPs need to vote against it- in the range of 20 and 30 before they start looking tight on the numbers, and if they lose, the big question is what do they do as an alternative as there's not many options."