“Everyone knows what sex is and you can’t change it,” said Susan Smith, co-director of For Women Scotland, which brought the case.

“It’s common sense, basic common sense and the fact that we have been down a rabbit hole where people have tried to deny science and to deny reality and hopefully this will now see us back to reality.”

Five judges ruled unanimously that the UK Equality Act means trans women can be excluded from some groups and single-sex spaces, such as changing rooms, homeless shelters, swimming areas and medical or counselling services provided only to women.

The ruling means that a transgender person with a certificate that recognises them as female should not be considered a woman for equality purposes.

The ruling brings some clarity in the UK to a contentious issue that has polarised politics in some other countries as well.

Justice Patrick Hodge said the British ruling “does not remove protection from trans people”, who are still protected from discrimination under UK law.

The case stems from a 2018 law passed by the Scottish Parliament saying 50 per cent of the membership of the boards of Scottish public bodies should be women.

Transgender women with gender recognition certificates were to be included in meeting the quota.

“Interpreting ‘sex’ as certificated sex would cut across the definitions of ‘man’ and ‘woman’ … and, thus, the protected characteristic of sex in an incoherent way,” Hodge said.

“It would create heterogeneous groupings.”

Trans rights advocates condemn the judgment

The campaign group Scottish Trans said it was “shocked and disappointed” by the ruling, saying it would undermine legal protections for transgender people enshrined in the 2004 Gender Recognition Act.

Maggie Chapman, a Green Party lawmaker in the Scottish Parliament, said the ruling was “deeply concerning” for human rights and “a huge blow to some of the most marginalised people in our society.”

“Trans people have been cynically targeted and demonised by politicians and large parts of the media for far too long,” she said.

“This has contributed to attacks on longstanding rights and attempts to erase their existence altogether.”

Groups that had challenged the Scottish government uncorked a bottle of champagne outside the court and sang, “women’s rights are human rights”.

“The court has given us the right answer: the protected characteristic of sex — male and female — refers to reality, not to paperwork,” said Maya Forstater of the group Sex Matters.

In 2022, an employment tribunal ruled that she had been the victim of discrimination when she lost out on a job after posting gender-critical views online.

The British government welcomed the ruling, saying it would provide clarity and confidence for women.

“Single-sex spaces are protected in law and will always be protected by this government,” it said.

Scotland’s semi-autonomous government said it accepted the judgment.

“We will now engage on the implications of the ruling,” First Minister John Swinney posted on X.

“Protecting the rights of all will underpin our actions.”

For Women Scotland had argued that the Scottish officials’ 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.

“Not tying the definition of sex to its ordinary meaning means that public boards could conceivably comprise of 50 per cent men and 50 per cent 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 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.”

Harry Potter author backed challenge

The women’s rights group counted among its supporters author JK 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.

Rowling said she was “so proud” of the “extraordinary, tenacious” For Women Scotland campaigners who took the case on a years-long battle through the courts.

Rowling wrote on X that “in winning, they’ve protected the rights of women and girls across the UK”.

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 UK 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.