Companies can legally block trans women from taking jobs intended only for women, the equalities watchdog has ruled.

The Equality and Human Rights Commission (EHRC) said that if an employer wanted to advertise for a woman-only job, that must only mean biological women or those who have received a gender-recognition certificate.

It would therefore be discriminatory to extend the definition of “woman” to men who self-identify as women.

Baroness Falkner, chairman of the EHRC, warned that her watchdog would take action against employers who breached the guidelines.

“As Britain’s equality regulator, we have a duty to promote and uphold Britain’s equality laws,” she said. “Those publishing job adverts must be familiar with their obligations under equality law. They can feel confident that our updated guidance will help them to comply with the law.

“Employers should also be aware that the EHRC will take action to uphold the Equality Act. Where we are made aware of potential misapplication of Schedule 9 provisions, we will continue to assess and take action to resolve these on a case-by-case basis.”

The guidance was welcomed by women’s rights campaigners, including Nic Williams, director of Fair Play for Women, who said: “We welcome this new guidance that makes clear that men who self-identity as women are not female and should not be recruited as such.

“We have been reporting rogue employers who misuse the law by advertising for ‘self-identifying women’ to fill women-only roles.

“That means males doing jobs reserved for females. That is unfair on the women who apply and the people who rely on a woman being in that role for reasons of privacy, dignity and safety.”

‘This sets out that self-ID is not the law’

The new guidance updates previous advice on placing or publishing advertisements for women-only jobs.

Examples of jobs where it is legal to restrict the role to a woman include a position in a refuge for rape survivors, or domestic violence hostel.

A discriminatory advert is one which restricts jobs, goods, services or facilities to people with a protected characteristic covered by the Equality Act 2010. They are unlawful except in very limited circumstances where the Equality Act specifically allows such a restriction.

The refreshed guidance provides greater clarity around “occupational requirements”, under Schedule 9 of the Equality Act 2010, where an employer can require a job applicant or employee to have a particular protected characteristic if it is necessary for the role.

The EHRC said it had seen evidence that some employers have incorrectly applied occupational requirement exceptions, and that there was a particular lack of clarity about the law where the protected characteristic is “sex”.

The updated guidance makes clear that where an occupational requirement relates to “sex”, the law says this means a person’s legal sex as recorded on their birth certificate or gender-recognition certificate.

Maya Forstater, chief executive of women’s rights charity Sex Matters, said: “We welcome this long overdue guidance from the EHRC which sets out that self-ID is not the law.

“Women’s-rights campaigners have been complaining to the EHRC for years about ads for female-only jobs and services that say they will accept men who identify as women.

“Employers can only restrict a service or a job to women with good reason, and it is never appropriate for men to try to access these services, regardless of how they dress, or whether they have a piece of paper claiming they’re women.”