Momentum founder Jon Lansman has suggested Jeremy Corbyn should not resign as Labour leader until the New Year if the exit poll forecasting catastrophic losses for the party proves accurate.
Mr Lansman, who set up the pro-Corbyn campaign group in 2015, said Labour winning just 191 seats - as the poll predicts - would be because "this was a Brexit election. Brexit was incredibly hard for us to navigate".
Appearing on ITV News' coverage of the election, he was asked whether Mr Corbyn should resign over the result, which would be Labour's worst in modern times.
"Jeremy Corbyn has to make those decisions himself. He's always been a reluctant leader," he said.
"I don’t think he’ll overstay his welcome, but he should be able to make decisions. I don't think we should rush into those things.
"Christmas is not far away. I don’t think decisions about this really need to be taken until the New Year."
He said it was a mistake for Labour leaders Gordon Brown and Ed Miliband to resign so shortly after losing elections in 2010 and 2015 respectively.
Mr Lansman was appearing alongside former Labour Home Secretary Alan Johnson, who said Labour had failed to attract working class voters and attacked Momentum, saying he wanted it "gone" from the party.
"The working classes have always been a disappointment for Jon and his cult," Mr Johnson said. "I want Momentum gone. Go back to your student politics.
"Corbyn was a disaster on the doorstep... everyone knew he couldn't lead the working class out of a paper bag."
Mr Lansman snapped back that Mr Johnson was showing the "anger he's always had about people like me".
Asked why the party's programme failed to speak to voters, Mr Lansman said: It hasn't on this occassion".
"Alan wants to kick [Momentum] out. I think that would be a disaster for the Labour Party," he said.
Early results have suggested the exit poll is accurate, including the historic loss of Blyth Valley, which Labour had held since its creation in 1950.