John McDonnell has been filmed calling the "plotters" who are trying to oust Jeremy Corbyn as leader as "f****** useless".
The shadow chancellor earned loud cheers at a pro-Corbyn fundraising event in London as he launched an foul-mouthed attack on the MPs seeking to replace his friend.
Minutes later, Corbyn reportedly told the same crowd: "We don’t throw abuse around."
His joke is unlikely to help relations in the bitterly-divided party.
McDonnell defended his remarks, insisting he was joking and that it was meant in a "light-hearted way".
"It was a stand-up comedy event. It was a joke. It was taken in a light-hearted way," McDonnell told BBC Radio 4's Today programme.
He insisted he was referring to a small group in the party who had never accepted Corbyn's leadership and not leadership challenger Angela Eagle.
McDonnell also joked that his late mother would "wash his mouth out with soap" for if she was still alive.