A contrite Nick Clegg drew both scorn and applause on Wednesday when he apologised for promising at the last election to oppose any increase in tuition fees, saying: "We made a pledge, we did not stick to it, and for that I am sorry."

The admission of error, rare in a modern politician, came before next week's Liberal Democrat party conference in Brighton at which his ability to take the party into the next election will be under scrutiny. In a released broadcast he conceded: "There is no easy way to say this."

It was stressed that Clegg was apologising for making the pledge to the National Union of Students before the election not to raise tuition fees, but not for the eventual decision by the coalition to lift the cap on fees to £9,000.

The deputy prime minister made the decision to confess in an attempt to win back a hearing from a public that he feels, at least until recently, had tuned him out because of its unrelenting anger over the broken tuition fee pledge.

Some of his advisers urged him against the apology, saying it would be taken as a sign of weakness, or merely disinterring an issue best left buried, but Clegg was convinced the confession was necessary after a summer tour of town hall meetings left him persuaded that the issue of the broken pledge would simply not go away, and was corroding trust.

In the broadcast he said: "It was a pledge made with the best of intentions – but we should not have made a promise we were not absolutely sure we could deliver. 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 would end up in government was in coalition with Labour or the Conservatives who were both committed to put fees up."

Promising to learn from his mistakes, he vowed: "I will never again make a pledge unless as a party we are absolutely clear about how we can keep it."

Clegg believes that with another coalition possible after the 2015 election, all three parties must be clearer about pledges that would survive coalition negotiations. He is aware some of the electorate will take his apology as a sign of vulnerability or the act of a desperate man at a time when his personal poll ratings are plumbing new depths, but he believes others will find his honesty refreshing. Clegg insists he will take his party into the next election and believes this move will help clear the path.

In an admission of how much damage he has suffered, he said he accepted his apology "won't be enough for everyone. But I owe it to you to be upfront about it. And I don't believe it should cast a shadow over everything else the Liberal Democrats are achieving in government. When we are wrong we hold our hands up. But when we are right we hold our heads up too."

Clegg's aides said he had been wanting to make this full apology for a long time, but had only sensed in the past few months that the electorate might be in a mood to listen. He did not tell his coalition partners of his plan, but did consult the business secretary, Vince Cable, the architect of the tuition fees policy.

As Clegg was touring the country in April 2010 signing the NUS promise not to increase tuition fees, his coalition negotiating team was planning to abandon the pledge.

Cable on BBC Newsnight conceded he had been told before the election that the student fees pledge was unaffordable and said he had been sceptical about its affordability. "It was an unwise commitment to have been made. We are collectively responsible. We all participated."

He insisted it was not a stunt: "It was part of a genuinely felt wish to assist the student population. We weren't able to carry through with it, but it was certainly deeper than a stunt."

Fellow Lib Dem David Laws said he supported Clegg's apology.

"Yes, of course I do, every Liberal Democrat MP has a collective responsibility, this was a decision not just by Nick Clegg and Vince Cable but by all of us in the Liberal Democrat party."

Laws, a close ally of Clegg who came back into government as schools minister this month after two years on the backbenches, denied that Clegg's making of a promise he knew he couldn't keep to win favour with his party represented a failure of leadership.

He told the BBC Radio 4 Today programme: "I don't think that's fair. He took on the party and told them what they didn't want to hear, which was that the pledge of abolishing tuition fees overnight was not affordable and we'd have to do it over two parliaments.

"He now regrets, and I now regret, and Vince Cable now regrets, that we didn't go further to point out how tough this would be, and we didn't make clearer in our manifesto the risk, given the fact that the other parties were committed to precisely the opposite policy.

"Both of them wanted to increase fees and that's why it would have been very difficult in coalition to deliver this policy."

An Ipsos/Mori poll showed satisfaction with Clegg has fallen from 31% to 23% in the last month, while dissatisfaction rose from 58% to 66%. This gives him his worst ever net satisfaction rating, -43. He even has a negative net personal rating of -8 among Liberal Democrat voters at the 2010 election. By contrast David Cameron and Ed Miliband rate at +42 and +21 respectively with their voters.

Shabina Mahmood, the shadow higher education minister, said: "Clegg's apology for promising not to raise fees, but not for £9,000 fees themselves, is too little too late for the students starting this week."

Tom Wood, chair of Liberal Youth, said: "The broken fees pledge was a cloud hanging over Liberal Youth, frustrating our efforts to highlight other important issues for young people. That cloud is now cleared."

Liam Burns, the NUS president, said: "Nick Clegg should be apologising to voters for breaking his pledge, not for making it in the first place."