John McDonnell has backtracked on Jeremy Corbyn’s pledge to pay off some historic student debt – downgrading it to an “ambition” for Labour.

At the general election, the Labour leader fuelled a huge surge in support among young people by promising help for students stuck with enormous debts from tuition fees.

Pledging to axe fees in future, he told NME magazine: “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.”

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But Mr McDonnell, the Shadow Chancellor, has now acknowledged the expected bill – around £100bn – is “a lot of money”.

He told the BBC’s Andrew Marr Show: “We will look at what we can do.”

Mr McDonnell added: “We are going to try to, it’s real ambition of ours – I don’t want to promise something we can’t deliver.”

However, she also admitted it was a “huge amount” and said the party would not commit to doing so “unless we can afford to”.

While Labour pledged to scrap university tuition fees in its manifesto, Mr Corbyn went further by suggesting existing debts could be wiped from the books.