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While the case centers on Scottish laws, the campaign group bringing the challenge, For Women Scotland (FWS), has said its outcomes could have U.K.-wide consequences for sex-based rights as well as everyday single-sex services such as toilets and hospital wards.

What's the case about?

The case stems from a 2018 law passed by the Scottish Parliament stating that there should be a 50% female representation on the boards of Scottish public bodies. That law included transgender women in its definition of women.

The women's rights group successfully challenged that law, arguing that its redefinition of “woman� went beyond parliament's powers.

Scottish officials then issued guidance stating that the definition of “woman� included a transgender woman who has a gender recognition certificate.

FWS is seeking to overturn that.

“Not tying the definition of sex to its ordinary meaning means that public boards could conceivably comprise of 50% men, and 50% men with certificates, yet still lawfully meet the targets for female representation," the group’s director Trina Budge said.

The challenge was rejected by a court in 2022, but the group was granted permission last year to take its case to the Supreme Court.

What are the arguments?

Aidan O’Neill, a lawyer acting for FWS, told the Supreme Court judges — three men and two women — that under the Equality Act “sex� should refer to biological sex and as understood “in ordinary, everyday language.�

“Our position is your sex, whether you are a man or a woman or a girl or a boy is determined from conception in utero, even before one’s birth, by one’s body,� he said on Tuesday. "It is an expression of one’s bodily reality. It is an immutable biological state.�

The women's right group counts among its supporters author J.K. Rowling, who reportedly donated tens of thousands of pounds to back its work. The “Harry Potter� writer has been that the rights for trans women should not come at the expense of those who are born biologically female.

Opponents, including Amnesty International, say barring transgender people who hold a gender recognition certificate from protections against sex discrimination conflicts with human rights principles.

Amnesty submitted a written intervention to the court, saying it was concerned about the deterioration of the rights for trans people in the U.K. and abroad.

“A blanket policy of barring trans women from single-sex services is not a proportionate means to achieve a legitimate aim,� the human rights group said.

The appeal hearing is expected to last two days with a judgement expected at a later date.