John McDonnell has denied watering down Labour pledges to reduce student debt, insisting it remains a "real ambition" to offer help.
The shadow chancellor said cancelling the current student debt would cost "about" £100 billion, recognising "it is a lot of money".
But he rejected suggestions that Jeremy Corbyn had promised to cancel the debt, adding the Labour leader had pledged to "try and tackle" the issue as the system is "collapsing".
Mr Corbyn, shortly before last month's general election and referring to those who "currently have a massive debt", told the NME he was looking at ways to reduce, ameliorate, lengthen the period of paying it off or "some other means of reducing that debt burden".
He added: "And I don't see why those that had the historical misfortune to be at university during the £9,000 period should be burdened excessively compared to those that went before or those that come after. I will deal with it."
Mr McDonnell, challenged about the pledge, told BBC 1's Andrew Marr Show: "The system is imploding, that's the problem.
"Half of the student debt, we now know, is not going to be paid back. So the system itself is falling apart. We've got to look at the system."
Mr McDonnell said Labour wants economists and educators to advise the party.
Asked if Labour will try to pay off the historic debt, Mr McDonnell replied: "We'll look at what we can do, it's a real ambition that we've got."
Presenter Andrew Marr said it sounded originally to be a promise, prompting the Labour MP to reply: "What Jeremy said is we recognise young people are coming out of college now with debts of £50,000 - they cannot even think about buying a house or get on the property ladder - so we've got to tackle that.
"But the issue is the system has got to be tackled anyway because it's falling apart."
Asked if a future Labour government would help students by dealing with some of their debt, Mr McDonnell replied: "We're going to try to. It's a real ambition of ours.
"I don't want to promise something we can't deliver."
Challenged by Mr Marr that Labour had made a promise before the general election, Mr McDonnell replied: "No, it wasn't a promise.
"What Jeremy said is we're going to try and tackle this issue. One of the reasons we've got to tackle it is because the system itself is collapsing."
Mr McDonnell said Labour remains committed to scraping tuition fees, adding: "If we can help and deal with the debt, we certainly will do.
"What we've said very, very clearly, because the system is collapsing, to be frank, whoever is in government has got to tackle this."
Education Secretary Justine Greening accused Labour of a U-turn.
She said: "Jeremy Corbyn and Labour have not been honest with young people. During the election campaign Mr Corbyn promised students he would wipe out tuition fee loans, at a cost of £100 billion.
"Now his chief lieutenants have U-turned on this commitment and young people will see it as a betrayal.
"Their position is in chaos, no one knows what they stand for or how they would pay for it."