Delegates gathered in Leeds to elect a National Policy Forum chairman, but the vote was halted amid complaints of insufficient time for the process.
Last night witnesses complained that supporters of hard Left movement Momentum urged the delay, fearing their preferred candidate would lose.
Veteran activist Ann Black was considered favourite to beat Momentum-backed Andi Fox to replace former MP Ann Cryer as NPF chairwoman.
Last month the group, with a controlling voice in Labour’s National Executive Committee, ejected Ms Black as chairwoman of Labour’s disputes panel, replacing her with Momentum director Christine Shawcroft.
And it again asserted itself yesterday with NEC chairman Andy Kerr jumping on stage to take over after a conference call of NEC officers voted 5-1 to stop proceedings.
The official reason was there had not been enough time to organise the vote, but sources say the move was motivated by a Momentum desire to control another seat in the ruling nine-strong NEC officers group – which would have been given to Ms Black automatically had she won.
One moderate MP said last night: “This morning symbolised the old school male union bullying that is determined to keep Jeremy Corbyn’s people in control, no matter how bad it looks to the outside world.
“Women delegates were coming up to [MP] Lucy Powell to thank her for trying to stand up to Andy Kerr, but the ex-union guy took the female chair of the policy forum into a room and kept her there until he got his way.”
Ms Powell said: “This smacks of old school control freakery, not new politics.”
And Ilford North MP Wes Streeting said: “The new version of democracy in the Labour Party: can’t win an election? Don’t hold one!”
A Labour Party source said: “For elections to be valid, seven days’ notice is required according to party rules.”