Unless the legislation is changed, the judgment is the final word on the defining question of the debate: what is a woman? The answer — at least in discrimination law — is that a woman is a person who was born female.

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The law in this area is complex, but the judgment is a model of clarity and provides a solid foundation for approaching consequential issues. It cannot cover everything, however, and since it was handed down there has been much public debate about its implications for everyday life. This has included challenges to its legitimacy and correctness.

Lively critical discussion is vital to civic life and democratic participation. Unfortunately, on this occasion much of it has been fuelled by misunderstanding, wishful thinking and distortion.

It has been said, for example, that competitive sex-segregated sports and single-sex facilities in workplaces, schools and services can operate on the basis of self-identified gender rather than biological sex. This was false before the Supreme Court judgment, and it is even more false now. Indeed if it were true, the appeal would not have been won.

Single-sex facilities are mandatory in workplaces and schools. The judgment has put it beyond doubt that the Equality Act, with other legislation, requires these to be provided according to biological sex.

Single-sex arrangements are not mandatory in competitive sports or services open to the public, such as gym changing rooms or refuges for victims of domestic violence. However, it is likely to be discrimination against women not to provide them where they are appropriate, and public authorities must pay particular attention to making adequate provision for women under the public sector equality duty.

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A service or competitive sport can be designated as single-sex if it meets conditions set out in the Equality Act. If that designation is made, then the service or sport must admit only people of one biological sex. Otherwise it ceases to meet those single-sex conditions, and must admit all members of the opposite sex.

It is not arguable that the law of indirect discrimination because of gender reassignment can be used to circumvent these principles.

Another assertion that has confidently been made is that an employer or service provider must justify excluding trans people from single-sex provision in accordance with their lived gender, and that this must be done on a case-by-case basis. Reference has been made to the legal justification test: “proportionate means of achieving a legitimate aim”.

Again, this is false. Operating a service or sport on a (biological) single-sex basis must be justified by showing that separating the biological sexes is a proportionate means of achieving a legitimate aim. Nothing further is needed to show that trans people should not be permitted to use the service in accordance with the sex in which they identify. Case-by-case decisions should not routinely be made: a single-sex rule must be predictable and consistent.

In fact, the judgment says that the Equality Act allows trans men (biological females) to be excluded from the women’s facilities, and trans women (biological males) to be excluded from the men’s. This might happen if, for example, a trans person looks so much like a person of the opposite biological sex that it would be disruptive to accommodate them in the single-sex service.

Undoubtedly this creates a double bind for trans people, and lawful solutions that preserve dignity and enable the full participation of trans people in public life must be found. An obvious one is to provide additional mixed-sex spaces alongside single-sex ones.

Criticising a Supreme Court judgment is not a novel activity, and citizens should be involved in debate about law reform. But lawyers, politicians and journalists must be mindful that their voices carry weight. It is irresponsible to make ill-informed or lazy challenges to the carefully reasoned decision of the country’s most exceptional legal brains.

The Supreme Court has decided what parliament intended by following established rules of statutory interpretation, which are explained in the judgment. What civil servants or politicians think they remember about what was said or intended during the drafting and passage of the Equality Act is entirely irrelevant to this exercise.

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Another claim being made is that the Supreme Court excluded trans voices, because it refused an application to intervene made by two trans individuals. But the Supreme Court does not hear evidence about lived experience; it considers legal arguments. A proposed intervener must show that they can make a distinctive contribution to the legal argument and assist the court with issues that go wider than their personal interest. Thus an individual is never likely to get permission, and it is advisable instead for applications to be made by representative organisations, such as charities or advocacy groups.

Of the many trans advocacy organisations in the UK, none applied to intervene. But their case was made thoroughly by leading practitioners acting for the well-established and reputable charity Amnesty and for the Scottish government.

Undermining the legitimacy of the judgment on such misconceived grounds helps nobody, and is all the more regrettable against the backdrop of misinformation that has been disseminated about the law relating to sex and gender from ostensibly trustworthy sources over many years.

The Equality and Human Rights Commission is working at pace to produce clear, authoritative guidance. Employers, service providers, sporting bodies and other duty-bearers under the Equality Act should urgently review their policies and practices, using reliable specialist advice that does not emanate solely from interested lobby groups. What is needed is constructive dialogue about how the law can work for everybody, based on a shared and accurate understanding of the law as it is, rather than the law as activists would prefer it to be.

Akua Reindorf KC is a commissioner for the Equality and Human Rights Commission