John McDonnell called Alastair Campbell a "f****** a*******" in a heated backstage argument following BBC's Question Time on Thursday night (15 September). The left-wing shadow chancellor and Tony Blair's former communications chief had agreed before the show they would not trade personal attacks.

But the uneasy truce shattered when McDonnell said Campbell and New Labour had "lost us five million votes" – only for the former New Labour press supremo to counter "you and yours are destroying" the party. According to two witnesses, neither man backed down after cameras stopped rolling as the angry, high-volume spat spilled over into the green room, anxiously watched by fellow panellists Conservative MP Anna Soubry and SNP MP Joanna Cherry, as well as BBC make-up artists and technicians. Also in the room was Campbell's 22-year-old daughter, Grace.

One witness in the room described how the hostile exchanges continued after the end of Question Time hosted by David Dimbleby – and said they feared the bad blood between the Labour 'big beasts' could have turned violent.

"It was a fiery debate followed by Alastair Campbell and John McDonnell going to the green room where they continued to shout at each other to the extent where it might even come to blows," the witness said.

"As part of the argument Alastair said 'You and yours are going to destroy the party'. John McDonnell was quite irritated and responded 'You're a f****** a*******'. John said it in front of everyone. Anna Soubry, Joanna Cherry and various other people were there but the MPs were quick to leave because of the atmosphere."

Campbell had McDonnell nodding in agreement early in the show when he attacked the government's plan to reintroduce grammar schools. The mood changed when Campbell suggested Labour was heading towards disaster under its current leadership.

McDonnell said Campbell's comments were "nauseating", sparking on-screen hostility that led Campbell to say: "I've come on here to be as nice to you as I possibly can and I'll tell you why: because I care about the Labour party, I really care about the Labour party and I worry that you and yours are destroying it. And what's more, I actually worry you don't even care."

Following the programme, Campbell was seen approaching Seb Corbyn – son of Labour leader Jeremy and aide to the shadow chancellor – to apologise for things boiling over and said he was responding to claims he lost the party millions of votes.

Campbell, who is an IBTimes UK columnist, tweeted after the show that McDonnell "hated" anyone associated with New Labour:

McDonnell also came under fire from Soubry, who said the shadow chancellor was a "nasty piece of work" for letting Labour MPs feel "terrorised" by his "cronies". McDonnell had not replied to a request for comment at the time of publication.

UPDATE (1550 BST 16 September):
- A source close to John McDonnell contacted IBTimesUK to deny that the incident took place.