A legal challenge over whether trans women can be regarded as female for the purposes of the 2010 Equality Act begins at the UK Supreme Court on Tuesday.

The action is the latest in a series of challenges brought by the campaign group For Women Scotland (FWS) over the definition of “woman” in Scottish legislation mandating 50% female representation on public boards.

The case centres on whether or not somebody with a gender recognition certificate (GRC) recognising their gender as female should be treated as a woman under the 2010 Equality Act.

The latest action is seeking to overturn a decision by the Scottish courts in 2023 which found that treating someone with a GRC as a woman under the Equality Act was lawful.

The Gender Representation on Public Boards (Scotland) Act 2018 is a piece of legislation intended to increase the proportion of women on public boards in Scotland.

In 2022, FWS successfully challenged the original act over its inclusion of trans women in its definition of women.

The Court of Session ruled that changing the definition of a woman in the act was unlawful, as it dealt with matters falling outside the Scottish Parliament’s legal competence.

Following the challenge, the Scottish Government dropped the definition from the act and issued revised statutory guidance – essentially, advice on how to comply with the law.

This stated that under the 2018 Act the definition of a woman was the same as that set out in the Equality Act 2010, and also that a person with a GRC recognising their gender as female had the sex of a woman.

FWS challenged this revised guidance on the grounds sex under the Equality Act referred to its biological meaning and said the Government was overstepping its powers by effectively redefining the meaning of “woman”.

However, their challenge was rejected by the Court of Session’s Outer House on December 13 2022.

The Inner House upheld that decision on November 1 2023 – but did grant FWS permission to appeal to the UK Supreme Court.

When the group’s legal argument was published ahead of the appeal last month, FWS director Trina Budge said: “Not tying the definition of sex to its ordinary meaning means that public boards could conceivably comprise of 50% men, and 50% men with certificates, yet still lawfully meet the targets for female representation.

“However, the ramifications of this case are much more far-reaching and all sex-based rights protected by the Equality Act are at risk.

“The stakes are high and the court’s decision will have consequences for everyday single-sex services such as toilets and hospital wards.

“It will determine whether a pregnant woman with a GRC is entitled to maternity leave, what it means to be same-sex attracted, and whether a man with a GRC’s entitlement to join a group of lesbians takes priority over their right to freely associate with only women.

“Trans rights are protected under the separate category of gender reassignment but to fully guarantee women’s rights it is increasingly clear that a consistent, biological and factual understanding of sex is the only workable solution.”

The Scottish Government said they were unable to comment on live legal proceedings.

The appeal before Lord Reed, Lord Hodge, Lord Lloyd-Jones, Lady Rose and Lady Simler is expected to last two days.