The United Kingdom's top court on Wednesday upheld an appeal by the campaign group For Women Scotland on whether transgender women are legally women under equality legislation, but it said trans people would not be disadvantaged by its landmark decision.

The unanimous judgment from five Supreme Court justices related to whether a trans woman with a gender recognition certificate (GRC), a formal document that gives legal recognition of someone's new gender, is protected from discrimination as a woman under Britain's Equality Act.

For Women Scotland had argued rights under the Equality Act should only apply based on a person's assigned sex at birth. It had challenged guidance issued by the devolved Scottish government that accompanied a 2018 law designed to increase the proportion of women on public sector boards.

Scottish ministers' guidance on that law stated that a trans woman with a full GRC was legally a woman.

"The terms 'women' and 'sex' in the Equality Act 2010 refer to a biological woman and biological sex, 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," said Patrick Hodge, deputy president of the Supreme Court.

Critics of the Scottish law had said its definition could impact single-sex services for women such as refuges, hospital wards and sports.

But transgender campaigners had said if the court ruled in favour of For Women Scotland, it could lead to discrimination against those with gender recognition certificates, especially over employment issues.

Hodge said the interpretation of the Equality Act "does not cause disadvantage to trans people, whether or not they possess a gender recognition certificate."

"Trans people have the rights which attach to the protected characteristic of gender reassignment," he said.

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.

Read the U.K. Supreme Court ruling: