The U.K. Supreme Court has unanimously decided that the legal definition of “woman” under the country’s equality laws refers to biological sex, excluding transgender women, even those with gender recognition certificates.

This landmark ruling, announced on April 16, 2025, marks a significant setback for transgender rights advocates and clarifies the interpretation of the Equality Act 2010.

According to the WSJ, The case originated from a challenge by For Women Scotland, a gender-critical campaign group, against the Scottish government’s guidance.

The guidance had suggested that transgender women with gender recognition certificates—legal documents recognizing a person’s acquired gender—should be treated as women under a 2018 law aimed at ensuring gender balance on public boards.

For Women Scotland argued that the Equality Act’s protections for women should apply only to those born female, asserting that biological sex is immutable.

The court’s 88-page judgment emphasized that the terms “woman” and “sex” in the Equality Act refer to biological sex, stating, “The concept of sex is binary—a person is either a woman or a man.”

Deputy President Lord Hodge, delivering the ruling, cautioned against viewing the decision as a victory for one group over another.

He stressed that transgender individuals remain protected against discrimination under the Act’s provisions for gender reassignment, as well as from harassment or discrimination based on their acquired gender.

The ruling has sparked varied reactions. Supporters of For Women Scotland, including prominent figures like J.K. Rowling, who has financially backed the group, celebrated the decision.

Rowling posted on social media, praising the “tenacious Scottish women” who brought the case, claiming it protects the rights of women and girls across the U.K. Conservative Party leader Kemi Badenoch called it a “victory for women who faced abuse for stating the obvious,” asserting that biological sex cannot be changed.

Conversely, transgender rights advocates expressed deep concern.

Scottish Trans, a charity, urged supporters not to panic but acknowledged the ruling reverses two decades of legal precedent recognizing trans women with gender recognition certificates as women.

The LGBT+ organization Stonewall labeled the decision “incredibly worrying,” warning of potential discrimination in employment and access to services.

Trans rights campaigner Charlie Gomersall lamented the loss of protections established in 2004, arguing the ruling undermines the dignity and safety of transgender individuals.

The decision could have far-reaching implications for single-sex spaces, such as women’s refuges, hospital wards, prisons, and sports, where biological sex may now take precedence in determining access.

The court noted that interpreting “sex” as certificated rather than biological could create inconsistencies, particularly for lesbian-only spaces or services, where a trans woman attracted to women might be classified differently.

It also highlighted practical challenges, as public bodies cannot easily verify whether someone holds a gender recognition certificate, which is private information.

The legal battle began in 2018 when For Women Scotland challenged the Scottish government’s inclusion of trans women in public board quotas.

After losing in Scottish courts, where Judge Lady Haldane ruled in 2022 that sex is “not limited to biological or birth sex,” the group appealed to the Supreme Court. The unanimous decision overturns that ruling, aligning the Equality Act’s interpretation with biological sex.

Critics of the ruling, including Amnesty International, argue it may erode transgender rights, potentially conflicting with human rights principles.

The Equality and Human Rights Commission, which supported the Scottish government’s stance, said it would need time to assess the ruling’s full implications.

Meanwhile, gender-critical activists like Maya Forstater of Sex Matters hailed the decision as a clear affirmation that sex refers to “reality, not paperwork.”

The U.K. government welcomed the ruling, stating it provides “clarity and confidence” for women and service providers.

However, the decision may prompt calls to amend the Equality Act or revisit the Gender Recognition Act 2004, which allows legal recognition of acquired gender.

For now, the ruling redefines the legal landscape for transgender women in the U.K., prioritizing biological sex in the application of equality laws.