The GMB Union has been called "institutionally sexist" after an independent report into sexual harassment within the organisation.
The report's author, Karon Monaghan QC, said "bullying, misogyny, cronyism and sexual harassment are endemic".
And she said the union's policies were not "sufficiently clear or robust".
The organisation's national president, Barbara Plant, apologised to members who had suffered, saying "real and lasting change is needed".
The investigation into the union, the UK's third-biggest, in terms of members, was launched in May.
Publishing her findings on Wednesday, Ms Monaghan said women were "underrepresented throughout the GMB's ranks", with branches "often organised in a way that deters women members' participation".
She said regional secretaries, who oversee "good governance" in different parts of the UK, were "and always have been, men" - and held "disproportionate power" in the union.
She added: "Many use that power to bully and manipulate members of the GMB's lay bodies."
Ms Monaghan also said there was a culture of "heavy drinking and late-night socialising, salacious gossip and a lack of professionalism".
She added: "Fundamental change is required if the GMB is to meet its promise to women in the GMB and to women in society more widely."
The QC made a list of 27 recommendations to the union to tackle its failures, including taking steps to increase representation of women at all levels, a bespoke complaints procedure for sexual harassment and an annual equalities audit.
For an organisation which is supposed to represent staff who may be bullied or harrassed in the workplace, it is clearly embarrassing that it has not apparently put its own house in order.
The report was prompted by an anonymous letter to the union's national president Barbara Plant by "concerned members and staff" of the GMB in the spring, which had complained about the handling, or mishandling, of complaints of harassment and assault.
The union has decided to publish Karon Monaghan's findings unexpurgated - and with the forthcoming election of a new general secretary, the GMB's most senior paid official, it has the opportunity to deliver the "transformational change" that its executive committee is now promising.
The union is affiliated to Labour - and another QC, Martin Forde, is currently chairing an inquiry into the party's culture, prompted by the leaking of a report which contained hitherto confidential emails and WhatsApp messages between staff.
It remains to be seen if he will be as straight-talking as Ms Monaghan.
Ms Plant, who has been the GMB's national president since 2018, said the report made "sad and difficult reading".
She said: "On behalf of GMB, I apologise to all those who have experienced sexual harassment or bullying within the union.
"It's clear that real and lasting change is needed for us to become a safe and inclusive place for all."
She said the union would now act on the report's recommendations, adding: "We are committed to achieving this transformational change."