Nick Clegg told the BBC that he was "absolutely wedded" to abolishing tuition fees but said he did not have a "magic wand where we can just say we can scrap all tuition fees tomorrow".
"We are saying we will do it over six years," said Clegg. "We will do it year group by year group starting first with final year students and working down over time. It is not a commitment we can deliver in one parliament."
In response to the announcement that the party no longer pledges to scrap university tuition fees in one Parliament, NUS President Wes Streeting said:
"For too long the Liberal Democrats' position on scrapping tuition fees has been about as clear as mud. If scrapping fees in the next Parliament is no longer an affordable promise, Nick Clegg needs to be crystal clear about what he will be promising students at the next general election. Woolly long term pledges won't wash and the Lib Dems should not take the student vote for granted.
"With record numbers of young people qualified to attend university and a severe squeeze on places, the Liberal Democrats need to be honest about where they stand on expanding student numbers and widening participation. If they believe in widening participation they must be transparent about how they will fund it. If not, they must explain to scores of young people why the Liberal Democrats support capping aspiration."