Jeremy Corbyn is fit enough to be prime minister well into his 70s if Labour wins the next general election, his closest ally has predicted.

There have been questions over the Labour leader’s health after civil servants briefed in June that he was not “physically or mentally” fit to lead the country.

Mr Corbyn called the civil servant comments, briefed to The Times and since investigated, a “farrago of nonsense” and the party said the 70-year-old ran and exercised every week, although confirmed he is receiving treatment for muscle weakness in his right-eye.

With an autumn general election looming, shadow chancellor John McDonnell has looked to allay any fears the public might have and predicted that his close ally is fit enough to last a five-year term and beyond in Downing Street.

“That’s for him to decide but I can’t see any reason why not – he’s perfectly fit, stamina of a young man,” said the Hayes and Harlington MP in an interview with the Financial Times.

If Labour wins a majority in any looming snap poll, the Islington North MP would become the oldest prime minister in more than 160 years.

The oldest person to serve as PM was William Ewart Gladstone who was 84 when he left office.

Mr McDonnell told the national newspaper that, under Mr Corbyn’s leadership, Labour was preparing to be even more radical than Clement Attlee’s post-war administration that established the NHS.

“We all admire what Attlee did …but I think we will go beyond it,” he told the FT.

Mr Corbyn’s right-hand man added: “People want change. Change is coming; as simple as that.

“I want a government that intervenes and that is going to intervene on a large scale.”

In preparation for taking over at the Treasury, Mr McDonnell’s team have been readying a host of policies since the publication of 2017’s manifesto.

“I’ve always said the left needs to be ready for government,” said the MP of more than two decades. “‘Be ready tomorrow, because things can happen.’ That time has come.”

The self-confessed Marxist plans to ban bankers’ bonuses, raise corporation tax and nationalise the railways as part of an interventionist government.

Yet, despite being painted as dangerous by successive Tory leaders, Mr McDonnell said private sector bosses were starting to move towards Labour due to the Government’s handling of Brexit.

“We’ve got business leaders coming to us looking for stability, which they’re not getting from the Tories,” he said. “It’s enhanced our relationship in that sense.”

And there are suggestions that Mr McDonnell is tapering his more left-wing announcements to ensure policies do not put off voters plucking for Labour at the next poll.

Earlier this year, Mr McDonnell threatened to delist stock market companies that did not meet strict environmental criteria but now he plays the suggestion down.

“I think rather than delist there’s other mechanisms we can look out for how to tackle it,” he said.

His utterances on supporting a four-day week also grabbed headlines but he suggests Labour will look to introduce flexible hours rather than force a three-day weekend upon businesses.

“For some people a four-day week is ridiculous because they’re desperate to get the hours just to survive at the moment,” he added.

“But there’s another group of people working all the hours God sends and it impacts on their family lives.”

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