Judges at the UK Supreme Court have unanimously ruled that a woman is defined by biological sex under equalities law.
It marks the culmination of a long-running legal battle which could have major implications for how sex-based rights apply across Scotland, England and Wales.
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, but campaign group For Women Scotland argued that sex-based protections should only apply to those born female.
The five judges said the ruling did not diminish trans women's protections against discrimination.
What does this mean?
The ruling means organisations will have legal backing to protect single-sex spaces such as changing rooms, lavatories and women's shelters on the basis of biological sex. Now every organisation in Britain has been told to revisit its equality policies.
The Equalities and Human Rights Commission (EHRC), the equality watchdog, will urgently update its guidance to organisations, and ministers are under pressure to set out what the decision will mean.
The government will also come under pressure to repeal the Gender Recognition Act, which allows people to obtain a gender-recognition certificate, after the “watershed” judgment rendered it “effectively defunct”.
What are people saying?
A trans woman who had gender reassignment at the age of 72 says the UK Supreme Court's ruling over the definition of a woman will have "dire consequences" for the safety of trans people.
Campaigner Kerrie Meyer, from Shetland, said it would allow "bigotry, transphobia, assault and untold life-threatening dangers to escalate".
Campaigners said organisations that allowed biological men into women’s spaces must immediately scrap the policies or face legal action.
Marion Calder, a director of For Women Scotland, said: “If there is a female sign on the door, that is now a single-sex space. That is crystal clear as a result of today’s ruling, GRC [gender-recognition certificate] or not.
“The many bodies which are operating self-ID policies must immediately scrap them. They are now illegal. If they do not, they will see multiple tribunals very quickly.”