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Jeremy Corbyn will quit as leader if Labour loses the next general election, his closest ally has vowed.

Shadow Chancellor John McDonnell said it was “inevitable” that both he and Mr Corbyn would resign if defeated by the Tories at the ballot box.

It is the first time any member of the leadership team have accepted there are circumstances beyond a vote of the Labour grassroots that would force them out.

Mr Corbyn has already survived losing a vote of no confidence among his own MPs, and refused to quit after failing to secure a ‘Remain’ vote in the EU referendum last month.

But asked on the BBC’s Marr Show if he and Mr Corbyn would resign following a general election defeat, Mr McDonnell said: “That would be inevitable. Of course it would – any Labour leader who loses an election usually goes.”

The last two Labour leaders - Ed Miliband and Gordon Brown - both resigned after losing general elections.

However Neil Kinnock did survive his 1987 failure to fight again - and lose again - in 1992.

It comes amid mounting speculation that new Prime Minister Theresa May could trigger a snap general election if Mr Corbyn wins this summer’s Labour leadership contest.

Polls show she is way ahead of Mr Corbyn in the polls, and she could be tempted to try to sidestep the new five-year fixed term Parliament Act and try to increase her fragile majority via a general election.

On Sunday Gordon Brown’s former aide Michael Dugher, now a prominent Labour MP, drew parallels with the situation in the summer of 2007 when Mr Brown took over as PM without a general election.

Mr Brown considered calling a snap vote that autumn, but ultimately shied away - and was punished at the ballot box three years later.

Mr Dugher said: “I’m convinced (Mrs May) won’t make the same mistake as Brown. I think she’ll be persuaded to go to the country sooner rather than later - maybe as early as October - if Jeremy Corbyn is still Labour leader after the September contest.”

(Image: Mirrorpix)

In a stark warning to Labour, Mr Dugher added: “If that happens I fear we could see the Tories get a majority of 100 seats. Labour could be looking at decades in the political wilderness.”

But Mr McDonnell insisted Labour is on the path to victory and pointed to the huge increase in party members since the 2015 election defeat.

“My argument is that we rebuild the Labour Party as a social movement and that’s what we’re doing,” the Shadow Chancellor said.

It comes amid extraordinary claims from Mr McDonnell’s former deputy, Seema Malhotra, that his aides tried to break into her office after she quit the shadow cabinet last month.

The former Shadow Chief Secretary to the Treasury has made a formal complaint to the Commons authorities after her private office was apparently accessed three times with an electronic key by McDonnell and Corbyn staff.

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“The implications of this are extremely serious,” Ms Malhotra said.

“This is a breach of parliamentary privilege and is a violation of the privacy, security and confidentiality of a member of parliament’s office.

“Furthermore, my staff... have felt harassed, intimidated and insecure and decided themselves it would be best to not leave anyone alone in the office.”

However Mr McDonnell said his office manager had simply been checking whether Ms Malhotra had moved out of the office yet after she quit the shadow cabinet in protest at Mr Corbyn’s leadership.

“I have now got a member of staff - she’s a widow with daughters, this is her sole income - she’s now worried she’s going to lose her job and face prosecution because it’s described as a break-in,” Mr McDonnell said.

“That’s just so distressing, it’s unacceptable. This has obviously been an error.”