A lawyer acting for a women’s rights campaign group has called on the UK Supreme Court to find sex is an “immutable biological state”.

Aidan O’Neill KC put forward the argument as he represented For Women Scotland (FWS) in its latest legal challenge against the Scottish Government over whether trans women can be regarded as female for the purposes of the 2010 Equality Act.

He told the court at the appeal hearing on Tuesday: “Our submission is that the court should find in favour of (For Women Scotland)… that in the Equality Act, sex just means sex, as that word and the words woman and man are understood and used in ordinary, everyday language, used every day in everyday situations by ordinary people.”

He said the Scottish ministers’ position that sex, man and woman in the Equality Act refer to “certificated sex” – as the sex on a person’s birth certificate whether or not amended by a gender recognition certificate (GRC) – is “just wrong and should be rejected by the court”.

Mr O’Neill called for the court to take account of “the facts of biological reality rather than the fantasies of legal fiction”, and to uphold the appeal.

He added: “Our position is your sex, whether you are a man or a woman or a girl or a boy is determined from conception in utero, even before one’s birth, by one’s body.

“It is an expression of one’s bodily reality. It is an immutable biological state.”

The action is the latest in a series of challenges brought by the campaign group 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 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 twice rejected by the Court of Session, which did grant FWS permission to appeal to the UK Supreme Court.

Mr O’Neill told the Supreme Court that FWS’s position is “not transphobic” and would be “preserving” rights rather than “closing down” rights.

He argued: “It’s simple and straightforward if you just stick to men being men, women being women and sex being sex.”

He said this provided a consistent reading across the Act and means a natal man who is now a trans woman with a GRC would not be defined as a woman under the Act.

Mr O’Neill said other interpretations mean “natal men gaining women’s rights via GRC and the other one involves natal women losing women’s rights as a result of GRC”.

He told the court Parliament could not have intended for the application of the Equality Act to result in “absurd or nonsensical outcomes”.

He argued the Scottish ministers’ approach would lead to “absurd and unjust consequences”.

In the Scottish ministers’ written submissions, they argued in this case “the term ‘woman’ where used in the 2018 Act includes a person issued with a full GRC in the acquired gender of female – and excludes
a person issued with a full GRC in the acquired gender of male”.

They argue their statutory guidance is lawful and the appeal should be refused.

The appeal before Lord Reed, Lord Hodge, Lord Lloyd-Jones, Lady Rose and Lady Simler continues.