Labour faced a fresh internal row after an election to a key policy position was postponed amid allegations of "bullying" by allies of Jeremy Corbyn.
The vote to elect a new chairman for the National Policy Forum (NPF) was put off amid complaints that insufficient time had been allowed for the process.
Reports suggested supporters of the left-wing Momentum group pressed for the delay amid concerns that their preferred candidate faced losing.
A source at the behind-closed-doors meeting in Leeds told the Press Association: "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."
Labour MP Lucy Berger said she was "ashamed" to witness the "disgraceful treatment" of acting NPF chairwoman Katrina Murray at the event
Fellow Labour MP Lucy Powell said the move "smacks of old school control freakery" in response to a HuffPost UK report of the delay.
The website reported veteran activist Ann Black - who lost the backing of Momentum - had been on course to defeat Andi Fox in the vote.
Momentum supporters now have a majority on the party's National Executive Committee (NEC) and already used that influence to oust Ms Black as chair of the party's disputes panel.
Manchester Central MP Ms Powell said: "The National Policy Forum is not a sub-Committee of the NEC and we can elect our own chair democratically.
"This smacks of old school control freakery. Not new politics."
Footage apparently shot inside the event showed Ms Powell arguing with NEC chairman Andy Kerr, who insisted that the vote should not go ahead.
A source said: "Women delegates were coming up to Lucy Powell to thank her for trying to stand up to Andy Kerr, but in the end the burly ex-union guy took the female chair of the policy forum into a room and kept her there until he got his way.
"This hideous spectacle has underlined that the new Momentum tendency may have the muscle to dominate but they risk destroying any hope we have of convincing the public we are a credible alternative to the Tories."
But Labour sources insisted that the delay in the election was necessary to comply with party rules.
"For elections to be valid, seven days' notice is required according to party rules," a source said.
"That's why the NEC ruled out an election at its first opportunity. NPF officers can't call an election as it would have breached rules.
"The NPF can't overrule the NEC."
The NPF was already a source of discontent for some in the Labour ranks because of the lack of a specific policy commission looking at Brexit,.
Some 17,000 emails have been sent to the party calling for the issue to be given its own commission with a full consultation of members' views.
Former minister Ben Bradshaw, a supporter of the Open Britain campaign, said: "I hope the entire National Policy Forum will agree that the absence of such a commission is problematic and inconsistent with our party's commitment to listening and responding to the views of our members.
"As we are reminded each and every day, Brexit is proving to be too costly and complicated to simply be swept under the carpet.
"The Government is all over the place. Labour must show we are able to provide answers and offer a clear and compelling alternative, and that means not being afraid to debate it and listen to the views of everyone within our party."