John McDonnell failed to explain Labour’s borrowing plans during a key broadcast interview, saying MPs have “iPads and advisers” for that. 

The Shadow Chancellor was grilled on the BBC Today Programme and was asked to talk about his own party’s plans to borrow around £50bn, but appeared to forget his sums. 

When presenter Mishal Husain pressed him about how much it would cost, he said it was “trite journalism” and “the wrong question” before repeating: “It would pay for itself.” 

He said: “The type of journalism where you go into an interview and they ask you a question on a particular figure is a trite form of journalism.

“It’s why we have iPads and advisors.”

McDonnell said borrowing was needed to pay for essential infrastructure like increased housebuilding.

He said: “What we would do is ensure that day-to-day spending was not paid for by borrowing. It will be paid for through our tax system, and that means stop the tax give-away to the rich and the corporations.

“That we would only invest for our infrastructure and that investment would pay for itself on the basis of the growth that we would achieve on those. And that’s a one-to-one return.

“Because, immediately that infrastructure puts more people back into work, they pay their taxes and as a result of that you are able to cover your costs.”

It is the Shadow Chancellor’s second shaky interview in as many days. 

When quizzed by Andrew Neil about the national debt on the BBC Daily Politics programme, he also slipped up.

Asked how much the UK spends paying interest on the national debt, McDonnell said he would send Neil “a note”. 

“I know the figure but I haven’t got it in front of me,” he said. 

Liberal Democrat leader Sir Vince Cable said: “It’s worrying that the shadow chancellor hasn’t done his sums. Or perhaps he has done his sums and he realises they’re so horrendous that he’s unwilling to acknowledge that his version of a Labour government would put the country into financial difficulties.” 

The Government spent £48bn in 2016/17 paying off national debt interest, accounting for around 3% of the UK’s estimate GDP.

Economic Secretary to the Treasury, Steve Barclay, said: “Labour refuse to come clean on how much taxpayers would have to pay for their borrowing binge.

“Labour would add hundreds of billions more to the country’s debt, meaning higher taxes on workers and less money for our schools and hospitals.”