The Good Law Project said it was taking the legal challenge after the Commission advised an employer that a trans woman with a gender recognition certificate (GRC) was required to use the men’s toilet at work.

The advice was given verbally two days before the landmark UK Supreme Court judgment which has prompted the Commission to ask the Northern Ireland courts to clarify the definition of “sex” in equality law.

After receiving correspondence from the Good Law Project and the woman, the Commission last week wrote to the employer to withdraw the advice.

However, the Good Law Project says it will continue with its lawsuit as the Commission is “still breaking the law”.

The lawsuit also contends that the paper published by the Commission in June, underpinning its decision to go to the courts for clarity on its interpretation of equality law when it comes to trans people, had failed to recognise a “fundamental protection for trans people”.

AL Goodbody is acting for the claimants — the unnamed trans woman and the Good Law Project — and has instructed Jason Coppel KC and Laura Curran BL.