Britain’s highest court has ruled the word “woman” refers to a “biological woman and biological sex” under an equality law in a ruling that is expected to upend the application of laws in the United Kingdom. 

In an 88-page ruling on Wednesday, the court said that the “concept of sex is binary” and that the terms “woman” and “sex” in the 2010 Equality Act refer to biological sex. 

Judge Lord Hodge said, “The unanimous decision of this court is that the terms woman and sex in the Equality Act 2010 refer to a biological woman and biological sex.”

“But we counsel against reading this judgement as a triumph of one or more groups in our society at the expense of another, it is not,” he added.

The judges were asked to determine if the word “sex” meant biological sex or “certified” sex.

The ruling was in response to a legal challenge to a 2018 law passed by the Scottish Parliament that stated that there should be 50 percent female representation on public boards. Individuals with a gender recognition certificate, or GRC, that recognizes them as having changed their gender, would have been included in that quota. 

A feminist group, For Women Scotland, brought the challenge to the law, raising the concern that if people could be recognized as changing their sex with a certificate, they would be allowed to claim protection from discrimination and access services or spaces meant only for women. 

However, the Scottish government argued that transgender individuals with a GRC should be afforded the same sex-based protection as biological women.

Lord Hodge said in the decision, “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. It would create heterogeneous groupings.”

The ruling means that individuals with a GRC will not be recognized as a woman under the law and it is expected to upend laws in the U.K. regarding single sex spaces such as changing rooms, homeless shelters, and counseling services for women.

Still, Lord Hodge said that the ruling gives transgender individuals “protection, not only against discrimination through the protected characteristic of gender reassignment, but also against direct discrimination, indirect discrimination and harassment in substance in their acquired gender.”

After the court’s ruling was released, the co-director of For Women Scotland, Susan Smith, said in a statement, “Everyone knows what sex is and you can’t change it.”

“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…hopefully this will now see us back to, back to reality,” she said. 

A spokesman for the U.K. government told the BBC the ruling will provide “clarity and confidence for women and service providers such as hospitals, refuges, and sports clubs.”

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

The conservative leader, Kemi Badenoch, posted on X, “Saying ‘trans women are women’ was never true in fact and now isn’t true in law, either. A victory for all of the women who faced personal abuse or lost their jobs for stating the obvious.”

“Women are women and men are men: you cannot change your biological sex,” she added. 

A transgender advocacy group, Scottish Trans, said it is “shocked and disappointed” by the ruling. A Green Party member of the Scottish Parliament, Maggie Chapman, called the ruling a “huge blow to some of the most marginalized people in our society.”

The author of the Harry Potter series, J.K. Rowling, wrote on X, “It took three extraordinary, tenacious Scottish women with an army behind them to get this case heard by the Supreme Court and, in winning, they’ve protected the rights of women and girls across the UK.”

The ruling received some praise in America from commentators who have advocated for keeping transgender individuals out of women-only spaces. Conservative commentator Megyn Kelly posted on X, “Thank God! Reason and REALITY prevail in the UK!”