The group plans to argue to the Equality and Human Rights Commission (EHRC) that the trade union is failing to adequately investigate sexual harassment and bullying claims, which could be a breach of its duties under the Equality Act.
The claims have emerged from multiple former GMB staff in recent days, with union sources saying a number of senior women have been given payoffs on leaving their roles.
The call for an independent inquiry comes four years after the GMB was described as "institutionally sexist" in a highly critical report by the barrister Karon Monaghan.
"Bullying, misogyny, cronyism and sexual harassment are endemic within the GMB," the report concluded at the time. "The culture in the GMB is one of heavy drinking and late-night socialising, salacious gossip and a lack of professionalism."
One GMB employee said: "Very little has changed since the Monaghan report, and in many people's views it has actually got worse."
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