Vince Cable has denied breaking promises on university fees, saying the Lib Dems are bound by the coalition deal - not pre-election pledges.

Their manifesto committed to scrapping tuition fees but they "did not win the election", the business secretary said.

But signing pre-election pledges to oppose any fee rises might have been the wrong "political judgement" by MPs, Mr Cable told BBC One's Politics Show.

Labour has attacked plans to lift the cap on tuition fees to up to £9,000.

And the National Union of Students said it was "insulting" for Lib Dems to deny they had made a promise.

Thousands of people took to the streets of London for a demonstration earlier this month, which flared into violence when a section of the march broke off to confront police and attack the Conservative party headquarters.

The National Union of Students (NUS) has also vowed to "chase down" any Liberal Democrat MPs who vote in favour of allowing universities in England and Wales to increase fees from the current cap of £3,290.

The NUS said many, including Mr Cable, had publicly signed its pre-election pledge to vote against any such rise, should they be elected in May.

'No betrayal'

But the business secretary, who is pushing through the reforms, insisted his party had not betrayed anybody.

"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.

A lot of the people who are protesting actually don't understand what's being proposed Vince Cable, Business Secretary

Asked whether it would have been better if he had not signed the pledge, he added: "From the point of view of political judgement 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 was bombarded with parliamentary questions on the subject earlier this month.

Labour's deputy leader Harriet Harman quoted him as having previously said that increasing tuition fees to £7,000 a year would be "a disaster" and described the new stance as "shameful".

Mr Clegg, the Lib Dem leader, insisted the coalition's policy - due to take effect from 2012 - was fair and that graduates who earned the least would pay less than currently.

Image caption Thousands of people have protested against the fee hike

However, he has admitted that he "should have been more careful" about signing the pledge in common with other Lib Dem MPs.

Amid expectations of further protests on Wednesday, Mr Cable told the BBC that many of those involved did not grasp what the government was proposing.

"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 year's time," he said.

"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."

NUS president Aaron Porter told the BBC students were angered by the Lib Dem's decision over tuition fees.

He said: "I accept that coalition brings with it compromise. But, their position was to abolish fees. I would suggest that a compromise position might be to try and keep them where they are.

"But to be the party that is overseeing a trebling of tuition fees, frankly, is a compromise too far. But also let's be clear, Vince Cable isn't actually saying that any more. He is saying that they didn't even make the promise.

"Well, I don't believe that and it's insulting to suggest otherwise."