Nick Clegg has issued a surprise apology for promising to oppose any increase in tuition fees and has pledged to learn from the "mistake".
"We made a promise before the election that we would vote against any rise in fees under any circumstances," he said on Wednesday.
"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."
The deputy prime minister issued the apology in a party political broadcast released early on Wednesday evening and will be seen as an attempt to draw a line under the issue ahead of the Lib Dem conference this weekend.
"I'd like to take this opportunity to put a few things straight," he says. "When I meet people around the country, it's obvious that many of you have strong – and pretty mixed – reactions to some of the things Liberal Democrats have done in government."
"Many of you tell me you're glad that at a time of real economic uncertainty, we put aside our political differences to provide our country with stable leadership.
"But, I also meet people who are disappointed and angry that we couldn't keep all our promises – above all our promise not to raise tuition fees.
He adds: "I shouldn’t have committed to a policy that was so expensive when there was no money around. Not least when the most likely way we’d end up in Government was in coalition with Labour or the Conservatives, who were both committed to put fees up.
"I also realise that isn’t the point. There’s no easy way to say this: we made a pledge, we didn’t stick to it – and for that I am sorry.
"When you’ve made a mistake you should apologise. But more importantly - most important of all – you’ve got to learn from your mistakes.
"And that’s what we will do. I will never again make a pledge unless as a party we are absolutely clear about how we can keep it."
The decision by the Lib Dems to support a rise in tuition fees after coming to power severely damaged the party's popularity among its supporters.
In an interview with the Huffington Post UK, to be published later this week and given prior to Clegg's apology, the Lib Dem deputy leader Simon Hughes said the party should have paid more attention to tuition fees.
"We could have got a deal that wouldn’t have given us the political problem we had," he says. "We could have done it differently."
It was, he adds, the biggest "political mistake" of the Lib Dems' two-and-half-years in government.
In the apology video, Clegg accepts that saying sorry would not 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," he says.
"When we’re wrong we hold our hands up. But when we’re right we hold our heads up too. We were right to leave the comfort of opposition to face the realities of government.
He adds: "That’s what my party believes in. That’s what I believe in. And, if we’ve lost your trust, that’s how I hope we can start to win it back."
The apology stands in stark contrast to an interview with the BBC in 2010 when Clegg insisted he would not be saying sorry.
"To govern is to choose particularly when there is not very much money and we have chosen and I am not going to apologise for this for one minute," he said at the time.
And the Lib Dem leader's apology is unlikely to satisfy his critics, who will no doubt point out he has apologised for making the pledge in the first place, not for subsequently breaking it.
Labour's deputy leader Harriet Harman said: "Instead of crying crocodile tears he should vote with Labour to bring these tuition fees down."
“If Nick Clegg does not back his words with action it will mean he is just weak and spineless," she added.
And Liam Burns, the president of the National Union of Students (NUS) said Clegg should be apologising to voters for breaking his pledge, not for making it in the first place.
“The Liberal Democrats’ commitment at the 2010 election was to abolish fees, a policy they’ve had for a decade. It’s what they said on the doorstep and in their leaflets, and every one of their MPs signed a pledge to back it up. It’s why many students and young people voted for the party," he said.
Public satisfaction with Clegg has fallen from 31% to less than a quarter according to a new poll, his worst ever ratings.
Approval ratings for all three party leaders dropped in the wake of the Olympics, the survey found.
Clegg's former director of strategy Richard Reeves, said the party needed to either back the Deputy Prime Minister in Brighton or sack him.
Reeves, who quit in July after working for the Deputy Prime Minister for two years, wrote in the New Statesman: "The mutterings have been growing louder for months, certainly since another bruising round of local election results in the spring.
"The question of Clegg's leadership has to be addressed. Indeed, given the party's current position, it would be irresponsible not to do so.
"Cards on the table: I think the party must stick with Clegg, and that Clegg must stick with both liberalism and coalition.
"Once the party caravan packs up on 26 September and heads inland after conference, the muttering has to stop. If the party is not to sack Nick Clegg then it must back him."
He also criticised him for not standing by a draft speech in which he called those who opposed gay marriage bigots. Clegg's office made the claim in a draft speech sent out to journalists only to recall the message and issue a second draft with the word removed.
The former aide added: "On gay marriage, the Liberal Democrats now have an open goal. Cameron is struggling to contain the harrumphing shire Tories. Labour is silent. It is a liberal cause."