Nick Clegg has admitted he "should have been more careful" about signing the pre-election pledge to oppose any increase in tuition fees.
He told ITV1's Daybreak "at the time I thought I could do it".
The Lib Dem leader and deputy PM said compromises had had to be made as part of the coalition deal.
But he added the planned changes were better than the existing fees regime and would help generations of "poor bright kids" to go to university.
Mr Clegg's comments came the day after a protest in central London against the proposal to raise the cap on annual fees charged by English universities to £9,000.
Student leaders say about 50,000 people took part in the protest, which became violent, with 35 arrests and 14 injuries, after the office complex which houses the Conservative Party HQ was stormed.
Before joining the Conservatives in a coalition after the general election, the Lib Dems had pledged to phase out tuition fees altogether over six years.
Many of the party's MPs, including Mr Clegg, signed a National Union of Students pledge not to vote for higher fees if re-elected in May.
But responding to a review of university funding by Lord Browne, the government has announced plans to allow some universities in England to charge up to £9,000 a year from 2012 - the current cap is £3,290.
'Head in the sand'
It was put to Mr Clegg on Daybreak that no-one would believe any pledge he made in future.
He acknowledged: "You need to be careful. I should have been more careful perhaps in signing that pledge at the time. At the time I thought we could do it."
"In politics as in life" there were times when you could not do what you wanted to, he said.
But rather than "put my head in the sand" and oppose any changes, he had worked to make the system "more progressive".
For Labour, shadow Commons leader Hilary Benn later told MPs that Mr Clegg had "known exactly what he was doing" in making the pre-election pledge.
It was not as if some "dodgy man approached him in the street" and persuaded him to sign the pledge, he said.
During prime minister's questions on Wednesday, deputy Labour leader Harriet Harman quoted Mr Clegg as having said before the election that increasing tuition fees to £7,000 a year would be "a disaster". "What word would he use to describe fees of £9,000?" she asked.
She suggested Mr Clegg had been "led astray" by the Conservatives, who had plans "to shove the cost of higher education on to students and their families".
A series of Labour MPs also got up to press Mr Clegg on the plans - which several of his own MPs, including former leaders Charles Kennedy and Sir Menzies Campbell, have said they will oppose.
But Mr Clegg said the government had come up with a "fair and progressive solution to a very difficult problem".
"The proposals we have put forward will mean that those who earn the least will pay much less than they do at the moment, those who earn the most will pay over the odds to provide a subsidy to allow people from poor backgrounds to go to university," he said.
He said the Lib Dems had to change policy because of the financial situation and compromises made in the coalition agreement.
"This is an extraordinarily difficult issue and I have been entirely open about the fact that we have not been able to deliver the policy that we held in opposition," he said.
He attacked Labour's record on university funding, saying they had opposed tuition fees in 1997 - then introduced them "a few months later" and had commissioned Lord Browne's review which they were now "trashing".
He said Labour's policy was for a graduate tax, a policy "half the front bench doesn't even believe in".