Jeremy Corbyn will resign as Labour leader if he loses a general election, says John McDonnell

Labour MP describes the shadow chancellor as 'the Gerald Ratner of politics. Sells you things he doesn't believe himself'

Jeremy Corbyn and the shadow chancellor, John McDonnell, will resign immediately if Labour loses the next general election, Mr McDonnell has announced.

It has been standard practice at every general election for a quarter of a century for the leader of the losing party to stand down, but they never promise to do so ahead of the contest, because by convention they never openly admit they might lose.

There is also no precedent that says the shadow chancellor has to fall on his sword in the event of defeat.

But when asked on BBC1’s Andrew Marr programme if he and Mr Corbyn would both resign after a defeat, Mr McDonnell was unusually direct.

“That would be inevitable," he replied.

Mr McDonnell also made an emotional appeal for an end to the infighting that is ripping Labour apart.

Looking directly into the camera and pointing, he said: “We have got to stop this now. There is a small group out there that are willing to destroy our party just to remove Jeremy Corbyn. We have got to stop them. We have got to unite.”

His performance on the Andrew Marr Show did not impress everyone.

The Labour MP Graham Jones tweeted: “John McDonnell, the Gerald Ratner of politics. Sells you things he doesn't believe himself. Train wreck of untruths on Marr.”

The last time the leader of either of the main parties lost a general election and stayed on was in 1987. When Neil Kinnock eventually resigned, after a second defeat in 1992, he was succeeded by his shadow chancellor, John Smith.

Mr Corbyn is currently facing a leadership challenge from the former shadow work and pensions secretary, Owen Smith, which he is expected to win after more than 180,000 people paid £25 to register as Labour supporters, the majority of whom are assumed to have done so to cast their votes for Mr Corbyn.

Mr Smith has written to Mr Corbyn asking for at least one hustings in every region of Britain in the hope of gaining ground.

But reuniting the party after seeing off Mr Smith would be a challenge for the embattled leader, when more than three-quarters of Labour MPs have declared they have no confidence in him.

There have also been widespread complaints about online abuse directed at Labour MPs, allegedly by Corbyn supporters. A letter has been sent to him from 45 female Labour MPs appealing to him to do more to end the “disgusting abuse”.

Mr McDonnell admitted there would have to be “mediated negotiations” between Mr Corbyn and Labour MPs if he is re-elected, and tried to soothe their anger by saying that Labour MPs are “good people (who) came into politics, like me, to change the world”.

He added that as “democrats” they would respect the outcome of a leadership contest.

Recently, Mr McDonnell was filmed at a supporters’ rally describing unnamed former members of Labour’s shadow cabinet as “f***ing useless”.