A Supreme Court hearing that will determine the definition of a woman begins today in a case with major implications for female-only spaces.

The case – which has been brought by gender-critical activists – will see five justices consider the question: ‘Is a person with a full gender recognition certificate which recognises that their gender is female, a ‘woman’ for the purposes of the Equality Act 2010?’

Feminist group For Women Scotland (FWS) will argue that trans women should not be able to fill roles meant for biological women under legislation designed to ensure equal numbers of men and women on company boards.

The Scottish government, which is contesting the case, insists the term woman includes a ‘person issued with a full GRC in the acquired gender of female’.

Gender-critical activists fear that, if they lose, it will render the definition of ‘woman’ in the Equality Act essentially meaningless.

They also argue that it will undermine women-only spaces and potentially trigger legal challenges over whether male-bodied criminals with a GRC should be held in a women’s prison.

A final ruling after the two-day hearing is not expected until next year. Harry Potter author JK Rowling has donated to FWS, saying last week: ‘If a man is a woman, there’s no such thing as a woman.’

Susan Smith, of FWS, said the case will have huge implications for women-only spaces and groups.

If FWS is unsuccessful ‘it will mean a lot of the advances and a lot of the progress that many women have made over the last few decades will be wiped out’, she added.

Helen Joyce, of human rights charity Sex Matters, said the case was ‘forcing the legal system to face up to the destructive ambiguity… concerning the legal definition of the word ‘woman’, which has done great harm to the rights of actual, flesh and blood women’.

The UK Supreme Court will hear a landmark case about the legal definition of a ‘woman’ today and Wednesday.

In 2018, Holyrood passed a law to increase the proportion of women sitting on the boards of public bodies to 50 per cent.

It defined ‘woman’ as including transgender women, whether or not they had a gender recognition certificate (GRC) changing their 𝑠e𝑥 in the eyes of the law.

The feminist group For Women Scotland (FWS) successfully challenged this at Scotland’s highest court, arguing it strayed beyond Holyrood’s powers.\

SNP ministers issued revised guidance on the issue, but this also stated that the definition of ‘woman’ included a trans person with a GRC.

FWS, who believe the Actwas meant to improve the representation of biological women on boards, not – as they would put it – men with GRCs, challenged the guidance.

FWS lost twice at the Court of Session and have now appealed to the UK Supreme Court. The Scottish Government has chosen to defend the case.

The dry court summary states the issue as: ‘Is a person with a full gender recognition certificate which recognises that their gender is female, a “woman” for the purposes of the Equality Act 2010?’

In practice, the result could have a profound effect on the gender debate in the UK.If FWS win, they hope it will cut through the ‘tortuous arguments’ of recent years and make biological 𝑠e𝑥 the defining quality for accessing services and spaces.

If FWS lose, they fear it will render the definition of ‘woman’ contained within the UK Equality Act essentially meaningless, undermine women-only services and spaces, and trigger legal challenges over whether male-bodied criminals witha GRC should be held in a women’s prison.

Lord Reed and four other justices are expected to deliver their opinion in the spring.