JOHN McDonnell has apologised if he came across as arrogant after he refused to say how much it would cost to service the borrowing needed to fund Labour's infrastructure plans.

The Shadow Chancellor said he had not wanted to get drawn into an argument that would undermine the fundamental idea that now was the time for the UK to borrow to invest.

His comments came as Barry Gardiner, his Shadow Cabinet colleague, said it would be foolish to try to predict the economy in 15 years' time, as he ruled out giving a date for when Labour would eliminate the budget deficit.

On Thursday, Mr McDonnell said it was "trite journalism" when asked how much extra would be needed to be spent on servicing debt under a Labour government, suggesting that specific figures could be got from iPads and advisers.

But, speaking on ITV's Peston on Sunday, he declared: "I'm sorry if people thought I was arrogant. What I was trying to do is not get dragged down into an argument, which prevents people realising that we actually do need to invest.

"And what happens is, that to get a headline…people will go 'how much is this going to cost?'"

The Shadow Chancellor went on: "I’m sorry if I annoyed people, the point I'm making is I didn't want the argument around a relatively low figure of what it would cost to undermine the real argument, which is this: that that investment wisely invested in our infrastructure, tackling our productivity problem, growing our economy, pays for itself."

Mr McDonnell was asked about an Institute for Fiscal Studies figure, which suggested the additional interest would be around £2 billion a year, minus any nationalisation projects.

"I can't predict it. I'm being straight with you here," insisted the London MP.

"As you know, government borrowing through bonds, etc at the moment…is anything between 0.4 and 1.89[per cent]. I can't tell when we go into government what that rate will be.

"What I can say is that the rates are at historic lows; this is the time to borrow," he added.

This week, the Office for Budget Responsibility, the independent forecaster, suggested that the budget deficit would not be wiped out until 2031.

Appearing on BBC One's Andrew Marr Show, when asked when the deficit would be eliminated under Labour's plans, Mr Gardiner said: "No, I'm not going to say that at all."

The Shadow International Trade Secretary, went on to say: "You've just heard one of our most eminent economists tell you that what, in your words, was 'we've drawn a line, and it could be on either side of it'.

"Anybody who wants to forecast what our economy is going to be like in 2031 here, 14, 15 years ahead of that date, when we have not even determined what the Brexit negotiations are going to look like, would be foolish."

Mr Gardiner said Labour would ensure the deficit would be reducing within five years, and that the economy would be growing under Jeremy Corbyn's plans, making the deficit less of a burden.

But Liz Truss, the Chief Secretary to the Treasury, said: “Labour can’t say when they’d eliminate the deficit because, with their eye-watering borrowing binge, they never would.

"And their Shadow Chancellor - the man who doesn't do numbers - again refused to come clean on how much that borrowing would cost taxpayers.”

She added: "Labour would saddle future generations with a mountain of debt, meaning higher taxes and fewer jobs. Once again, ordinary working people would pay the price.

“Our balanced approach as set out in the Budget - reducing debt while investing in public services - will build a Britain fit for the future.”