*This is being reported by The Guardian

The UK supreme court has issued a historic and definitive ruling that the terms “woman” and “sex” in the Equality Act refer only to a biological woman and to biological sex.

In a decision that delighted gender-critical activists, five judges ruled unanimously that the legal definition of a woman in the Equality Act 2010 did not include transgender women who hold gender recognition certificates (GRCs).

The judgment could have far-reaching ramifications and lead to greater restrictions on the access for trans women to services and spaces reserved for women. It prompted calls for the UK’s laws on gender recognition to be rewritten.

The UK government said the ruling brought “clarity and confidence” for women and those who run hospitals, sports clubs and women’s refuges.

A spokesperson said: “We have always supported the protection of single-sex spaces based on biological sex. Single-sex spaces are protected in law and will always be protected by this government.”

The case was brought to the supreme court by the gender-critical campaign group For Women Scotland, which is backed financially by JK Rowling, after two Scottish courts rejected its arguments that the Equality Act’s definition of a woman was limited to people born biologically female.

Lord Hodge, the deputy president of the court, said the Equality Act was very clear that its provisions dealt with biological sex at birth, and not with a person’s acquired gender, regardless of whether they held a gender recognition certificate.

That affected policymaking on gender in sports and the armed services, hospitals, as well as women-only charities, and access to changing rooms and women-only spaces, he said. However, trans women still have equal pay rights as women, and could have the right to be treated as women in some situations.

In its 88-page judgment, the court said that while the word “biological” did not appear in the definition of man or woman in the Equality Act, “the ordinary meaning of those plain and unambiguous words corresponds with the biological characteristics that make an individual a man or a woman”.

If “sex” did not only mean biological sex in the 2010 legislation, providers of single-sex spaces including changing rooms, homeless hostels and medical services would face “practical difficulties”, it said.

The justices added: “Read fairly and in context, the provisions relating to single-sex services can only be interpreted by reference to biological sex.”

The ruling represents a significant defeat for the Scottish government. For Women Scotland had initially challenged legislation that allowed trans women with a GRC to sit on public boards in posts reserved for women.

Scotland’s first minister, John Swinney, said his government accepted the court’s judgment. He said it clarified the limits of the Gender Recognition Act 2004, which introduced gender recognition certificates for trans people.

“We will now engage on the implications of the ruling,” he said. “Protecting the rights of all will underpin our actions.”

The Scottish government defended its actions in the case, which it said were always guided by the Equality and Human Rights Commission’s advice. It said it would now engage with UK ministers and with the EHRC to look at the ruling’s implications, since the legislation involved was passed by Westminster.

Trans rights campaigners urged trans people and their supporters to remain calm about the decision.

The campaign group Scottish Trans said: “We are really shocked by today’s supreme court decision, which reverses 20 years of understanding of how the law recognises trans men and women with gender recognition certificates.

“We will continue working for a world in which trans people can get on with their lives with privacy, dignity and safety. That is something we all deserve.”

Sacha Deshmukh, the chief executive of the human rights group Amnesty International UK, which joined with the Scottish government in the supreme court case, said the decision was “clearly disappointing”.

“There are potentially concerning consequences for trans people, but it is important to stress that the court has been clear that trans people are protected under the Equality Act against discrimination and harassment,” he said.

“The ruling does not change the protection trans people are afforded under the protected characteristic of ‘gender reassignment’, as well as other provisions under the Equality Act.”

Susan Smith, a co-founder of For Women Scotland, said the legal action had been “a really, really long road”. “Today the judges have said what we always believed to be the case, that women are protected by their biological sex,” she said.

“Sex is real and women can now feel safe that services and spaces designated for women are for women and we are enormously grateful to the supreme court for this ruling.”

In a social media post, JK Rowling said: “It took three extraordinary, tenacious Scottish women with an army behind them to get this case heard by the supreme court,” adding: “I’m so proud to know you.”

Hodge, the deputy president of the court, said it believed the position taken by the Scottish government and the EHRC that people with gender recognition certificates did qualify as women, while those without did not, created “two sub-groups”.

This would confuse any organisations they were involved with. A public body could not know whether a trans woman did or did not have that certificate because the information was private and confidential.

And allowing trans women the same legal status as biological women could also affect spaces and services designed specifically for lesbians, who had also suffered historical discrimination and abuse.

Kishwer Falkner, the chair of the EHRC, said it was pleased the ruling had dealt with its concerns about the lack of clarity around single-sex and lesbian-only spaces, but would need time to fully understand its implications.

“We are pleased that this judgment addresses several of the difficulties we highlighted in our submission to the court, including the challenges faced by those seeking to maintain single-sex spaces, and the rights of same-sex attracted persons to form associations.”