A report into a brawl involving leaders from a train drivers' union Aslef is urging members to stamp out a "self destructive culture".
Aslef's general secretary Shaun Brady faces disciplinary action
A leaked copy of the internal document by a three-member panel says the union is "highly factionalised".
The panel found the president and the general secretary should face disciplinary action but three other staff should not have been suspended.
The incident took place at the union's Hampstead headquarters on 20 May.
The panel who set up the inquiry, voiced "grave concern" over what happened and the effect it was having on the union.
It warned members that it is "imperative that Aslef take steps to turn itself around" and "replace what appears to be a self-destructive culture with one of which its members can be proud".
All the top leaders of the union had been invited to the private barbeque, according to BBC labour affairs correspondent, Stephen Cape.
But the leaked report shows that president Martin Samways became verbally aggressive with staff particularly with one woman, Julie Atkinson.
The new Aslef general secretary Shaun Brady attempted to physically remove his colleague when the woman was hit in the mouth by the president.
No firm conclusion was reached on whether the blow was deliberate.
But the report said the behaviour was totally unacceptable.
The two men were believed to have fallen onto the ground in what was described as a "brawl, fight or wrestling".
But the inquiry team said it went well beyond self defence.
The panel also complained that the inquiry was hampered because memories of some of the witnesses were clouded by alcohol and others lacked credibility.
It went on to criticise the acting general secretary for suspending three members of staff when it was not necessary.
TUC general secretary Brendan Barber told BBC Radio Five Live: "It is an episode that has caused a lot of damage and a lot of lurid headlines and no one can get any pleasure from that."
He said there were some "very, very good people" in Aslef who wanted to move on from the incident and concentrate on helping their members.
"The important thing is that the union have dealt with the issue and are dealing with the issue in a very proper way," he added.