Britain's highest court ruled on Wednesday that only biological and not trans women meet the definition of a woman under equality laws, a landmark decision greeted with concern by trans supporters but welcomed by the government as bringing clarity.

The much-anticipated ruling centred on whether a trans woman with a gender recognition certificate (GRC), a formal document giving legal recognition of someone's new gender, is protected from discrimination as a woman under Britain's Equality Act.

The decision confirms that single-sex services for women such as refuges, hospital wards and sports can exclude trans women, clearing up legal ambiguity. Transgender campaigners said the decision could lead to discrimination, especially over employment issues.

"The unanimous decision of this court is that the terms 'women' and 'sex' in the Equality Act 2010 refer to a biological woman and biological sex," Deputy President of the Supreme Court Patrick Hodge said.

"But we counsel against reading this judgment as a triumph for one or more groups in our society at the expense of another - it is not."

Transgender rights have become a highly political issue in Britain and other parts of the world. Some critics say the conservative right has weaponised identity politics to attack minority groups, while others argue that liberal support for transgender people has infringed on the rights of biological women.

In the United States, legal challenges are underway after President Donald Trump issued executive orders that include barring transgender people from military service.

Scottish guidance

Wednesday's judgment in Britain followed legal action by a campaign group, For Women Scotland (FWS), against guidance issued by the devolved Scottish government that accompanied a 2018 law designed to increase the proportion of women on public sector boards.

The guidance said a trans woman with a gender recognition certificate was legally a woman. FWS, which was backed by lesbian rights groups, lost its case in the Scottish courts, but the Supreme Court ruled in its favour.

"Today the judges have said what we always believed to be the case: that women are protected by their biological sex, that sex is real and that women can now feel safe that services and spaces designated for women are for women," Susan Smith, co-director of FWS, told cheering supporters outside court.

Britain's Labour government said the Supreme Court's decision would bring clarity for hospitals, refuges and sports clubs.

"Single-sex spaces are protected in law and will always be protected by this government," a government spokesperson said.

In an example of the ruling's potential impact, a Scottish health organisation that is being sued by a nurse it suspended over her response to a trans woman using a female changing room said it had noted the judgment.

"We will now take time to carefully consider the judgment and its implications," a spokesperson for NHS Fife said.

Harry Potter author J.K. Rowling, who has been vocally gender critical, was among those who welcomed the decision.

"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," Rowling said on X.

The Supreme Court said trans people - whether trans women or men - would not be disadvantaged by its decision as the Equality Act afforded them protection against discrimination or harassment.

Trans rights campaigners said the ruling had worrying implications.

"Today is a challenging day, and we are deeply concerned at the widespread, harmful implications of today's Supreme Court ruling," a consortium of LGBT+ organisations, including prominent group Stonewall, said in a statement.

"We need to take the time to digest the full implications of the ruling and to understand what this will mean on both legal and practical levels ... it is important to be reminded that the Supreme Court reaffirmed that the Equality Act protects trans people against discrimination."

Trans woman and campaigner ‪Ellie Gomersall said it was "another attack on the rights of trans people to live our lives in peace".

Legal experts said the ruling showed equality legislation might need to be urgently updated to ensure trans people were protected.

Phillip Pepper, employment partner at law firm Shakespeare Martineau, said the court's decision could "create further division and increase tensions" in the short term.

"However, it will offer long-term clarity for businesses which have been left to interpret ambiguous, contradictory legislation on their own until this point, potentially landing in hot water as a result," he said.