Business Secretary Vince Cable has denied breaking promises on university tuition fees, insisting the Liberal Democrats' pre-election pledge to oppose any rise was not binding.
He acknowledged that signing the pledge might have been the wrong "political judgment" but maintained that the coalition's plans now to increase fees did not reflect badly on the Lib Dems' trustworthiness.
In a pre-recorded interview with BBC1's Politics Show, to be broadcast today, Dr Cable said the Lib Dems "haven't betrayed anybody" and that the coalition agreement struck with the Tories was their only binding commitment.
"We didn't break a promise. We made a commitment in our manifesto, we didn't win the election. We then entered into a coalition agreement, and it's the coalition agreement that is binding upon us and which I'm trying to honour," he said.
Asked whether it would have been better if he had not signed the pledge, he added: "From the point of view of political judgment it may well have been, but it's not an issue of trust.
"We and the Conservatives separately made a whole series of commitments in our manifesto and outside it.
"We haven't been able to carry all of them through, partly because we have a coalition and have had to make compromises and partly because we're still in the middle of this appalling financial situation."
Deputy Prime Minister Nick Clegg has also admitted that he "should have been more careful perhaps" about signing the pledge in common with other Lib Dem MPs. During the General Election campaign, Mr Clegg said raising tuition fees would be a "disaster".
Government plans to allow universities to charge up to £9,000 a year in tuition fees have been met with fury by existing students and those hoping to enter higher education in the future.
Millbank Tower, the home of Conservative Party headquarters, became a focus for anger and the scene of violent clashes during demonstrations earlier this month.
Amid expectations of further protests on Wednesday, Dr Cable suggested many of those involved did not grasp what the Government is proposing.
"I think a lot of the people who are protesting actually don't understand what's being proposed," he said.
"It doesn't actually affect them - we're talking about a system of graduate contribution that will only affect people who start going to university in a couple of years' time.
"If they are concerned for the next generation what I think they do need to understand is that we're making the system significantly fairer, making it much more attractive for part-time students and for graduates on low incomes."