The court sided with campaign group For Women Scotland, which had brought a case against the Scottish government on the basis that sex-based protections should apply only to people born female.

Deputy President Lord Hodge, Lady Rose and Lady Simler gave a joint judgment, with which the other justices agreed.

Aidan O’Neill KC and Spencer Keen appeared for the appellants, For Women Scotland, and were instructed by Balfour and Manson. Ruth Crawford KC and Lesley Irvine, for the respondents, were instructed by the Scottish Government Legal Directorate.

The Scottish government had argued that transgender people with a gender recognition certificate (GRC) are entitled to the same sex-based protections as biological women.

The court was asked to decide on the proper interpretation of the Equality Act. Lord Hodge said the case turned on how the words “woman” and “sex” are defined in the legislation.

He told the court: “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.”

The judge also said that the legislation gives transgender people “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”.

The justices had been asked to rule on what is meant by “sex” in law, whether it means biological sex, or legal, “certificated” sex as defined by the Gender Recognition Act 2004.

The Scottish government said the 2004 legislation was clear that obtaining a GRC amounts to a change of sex “for all purposes”.

But For Women Scotland argued for a “common sense” interpretation of “man” and “woman”, telling the court that sex is an “immutable biological state”.

The judgment stated that a certificated sex interpretation would weaken the protections given to those with the protected characteristic of sexual orientation – for example by interfering with their ability to have lesbian-only spaces and associations.

It also pointed out that the practical problems that would arise under a certificated sex approach are clear indicators that this interpretation is not correct. The court rejected the suggestion of the Inner House that “woman” and “sex” can refer to biological sex in some sections of the Equality Act and certificated sex in others. The meaning of “sex” and “woman” must be consistent throughout the Act.