Nick Clegg has apologised for breaking a Liberal Democrat election promise to oppose any increase in university tuition fees.

Speaking before the Liberal Democrat Party Conference in Brighton, the Deputy Prime Minister said he was sorry his party had broken its pledge ahead of the 2010 general election to vote against a hike in fees for students.

His mea culpa came amid reports he had refused to use an opt-out clause in the Coalition Agreement whereby Liberal Democrat MPs would abstain in votes as the controversial legislation increasing fees to up to £9,000 a year made its way through the Commons.

He said he was sorry but not for the increase in tuition fees itself. Instead, he was sorry the Liberal Democrats ever opposed the increase as they had not costed the policy properly.

Many voters he had met were "angry and disappointed" at the party's decision to renege on its opposition to tuition fees, he says, adding: "To those people, I say this: we made a promise before the election that we would vote against any rise in fees under any circumstances.

"But that was a mistake. It was a pledge made with the best of intentions - but we shouldn't have made a promise we weren't absolutely sure we could deliver.

"I will never again make a pledge unless as a party we are absolutely clear about how we can keep it. I accept that won't be enough for everyone. But I owe it to you to be up front about it. And I don't believe it should cast a shadow over everything else the Liberal Democrats are achieving in Government."

The apology comes as some within the party have started to question whether Mr Clegg is the right leader for the party going in to the 2015 general election.

In a short statement, Labour Deputy Party Leader Harriet Harman accused Mr Clegg of "crying crocodile tears".

She said: "This was not just the small print of his manifesto, this was Nick Clegg's key election promise when he asked people to vote for his party. It is not good enough for him to just brush that promise aside.