HMRC has been accused of disobeying the Supreme Court gender judgment after allowing trans women to use female toilets.
The tax authority stated in guidance published on its staff intranet that employees could "use the toilet appropriate to your new gender".
Gender-critical campaigners have hit out, saying it dismisses the Supreme Court ruling in April.
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Judges ruled that the terms "woman" and "sex" in the 2010 Equality Act "refer to biological woman and biological sex".
The HMRC document said: "Gender reassignment - Getting ready for your first day in your new role," explains how employees should feel respected in the workplace after transitioning.
It reads: "You can then expect: People to treat you in your new identity, use the toilet appropriate to your new gender, people to call you by your new name."
Campaigners claimed the HMRC's guidance on the use of toilets was "unlawful".
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HMRC guidance stated: 'Use the toilet appropriate to your new gender'
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Director of campaigns at Sex Matters, Fiona McAnena, said: "It seems HMRC is still following Stonewall law, which demands that trans-identifying men are given access to women's toilets and changing rooms."
Speaking to The Telegraph, Miss McAnena continued: "The message to every female staff member at HMRC is that their rights and privacy don't matter. If a man says he is a woman, his wishes come ahead of their rights and needs. This isn't just bad policy; it is unlawful.
"The statement that trans women are women, and the implication that this gives them the same rights as women, insults every woman who files a tax return."
Miss McAnena explained that their is a mainstream belief that men cannot become women is protected in law and the HMRC has no business declaring otherwise.
Miss McAnena added: "HMRC and every Government department should be leading the way in complying with the law.
"Instead, they're risking legal action from their female staff for discrimination and harassment."
Susan Smith, one of the co-directors of Women Scotland, who was seen celebrating the Supreme Court's ruling in April, said: "Of all people, they should know that taxpayers will not be impressed if they have to foot the inevitable tribunal costs."
She continued: "All Government bodies now need to rip up unlawful policies and start again. Toilets, showers and changing facilities may be mixed-sex where they are in a separate room lockable from the inside."
Co-director of For Women Scotland Susan Smith (L) celebrates after terms 'woman' and 'sex' in the Equality Act refer to a biological woman and biological sex, according to a Supreme Court ruling | PAAccording to reports, new EHRC guidance to be considered by equalities minister, Bridget Philipson, will tell public bodies to ban transgender women from female-only spaces.
Multiple public bodies, including the NHS, have not imposed such a ban until guidance is issued.
The EHRC referred to a statement from its chair, Baroness Falkner, when asked if HMRC was complying with its guidance.
It said: "The law it [the judgment] sets out is effective immediately. Those with duties under the Equality Act should be following it and taking specialist legal advice where necessary."
A HMRC spokesman said: "We comply fully with all existing statutory guidance. As and when this guidance is updated, we will of course comply with it as legally required."