Susan Smith (L) and Marion Calder, directors of the "For Women Scotland" group, speak to media outside the U.K. Supreme Court, April 16, 2025 in London, England. Dan Kitwood/Getty

The U.K. Supreme Court ruled Wednesday that a British equalities law defines a woman as someone born biologically female. Justice Patrick Hodge said five judges at the court had ruled unanimously that "the terms 'woman' and 'sex' in the Equality Act refer to a biological woman and biological sex."

The ruling on the interpretation of the 2010 law means a transgender person with a certificate that recognizes them as female should not be considered a woman for equality purposes under the legislation. But the court added that its ruling "does not remove protection from trans people," who are "protected from discrimination on the ground of gender reassignment."

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.

For Women Scotland (FWS), a women's rights group, had challenged that law, arguing that its redefinition of woman went beyond parliament's powers. But Scottish officials then issued new guidance stating that the definition of woman included someone with a gender recognition certificate. FWS successfully sought to overturn that.

The ruling by the top U.K. court comes amid efforts by the Trump administration to bar transgender people from serving in the U.S. military and participating in women's sports, and to prohibit gender-affirming care for minors — many of which are currently facing legal challenges in U.S. courts.

FWS had said the outcome of the case could have consequences in Scotland, England and Wales for sex-based rights as well as single-sex facilities such as toilets, hospital wards and prisons.

"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 previously.

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.

Aidan O'Neill, a lawyer 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. "It is an expression of one's bodily reality. It is an immutable biological state."

The women's right group counted 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 vocal in arguing that the rights for trans women should not come at the expense of those who are born biologically female.

Opponents, including Amnesty International, said excluding transgender people from sex discrimination protections conflicted with human rights laws.

Amnesty submitted a brief in 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.

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