How will this showdown over gender laws play out?

Scotland's parliament recently passed a bill making it easier for people in Scotland to "self-identify" and reassign their gender. Before the bill gains royal assent and becomes law it needs to be consistent with UK law as determined by Westminster. UK ministers have indicated they will block the legislation.

- SEAN O'GRADY

Why is a Scottish law about gender reassignment causing so much trouble?

The issue of trans rights is controversial in any setting, as demonstrated by the extraordinary way trans people and JK Rowling respectively have found themselves the target of so much vile abuse, especially on social media.

Trans rights have particularly divided the left, as we see from the way Keir Starmer distanced himself from MSPs on the subject of the age that someone can legally change their gender. But it's also been an issue on the right; Penny Mordaunt's Tory leadership campaign was hampered by her own past progressive statements that trans women were women. Suddenly, the question "can a woman have a penis?" became the most dangerous in British politics.

So during the final voting on the Gender Recognition Reform (Scotland) Bill in Holyrood, there were loud and graphic scenes in the public gallery, and much-impassioned argument inside and outside the chamber. The bill was carried by 86 votes to 39, with cross-party support, but it is a polarising question in Scotland as elsewhere.

Overlaying the arguments about the rights of trans people are some equally sensitive rights of Scotland and the intense resentment among some of a Scottish law being interfered with by a British government in London. This hasn't happened since devolution was enacted some 25 years ago. There's a lot of anger around.

What are the legal issues?