The Scottish and UK governments will face each other in a court clash over Holyrood’s reforms to the gender recognition process in Scotland.
The Gender Recognition Reform Bill passed in December after lengthy debates at the Scottish Parliament, often amid divisive political rhetoric.
It has been discussed for years, with strong opinions for and against changing the 2004 Gender Recognition Act.
UK ministers stepped in to stop the Bill becoming law.
Here, the PA news agency answers some of the key questions about the legislation, and what comes next.
The main element of the Bill is to make it easier for trans people to obtain a gender recognition certificate (GRC) by removing the requirement for a diagnosis of gender dysphoria.
It will also lower the minimum age for applicants from 18 to 16 and drop the time required for an applicant to live in their acquired gender from two years to three months – six for people aged 16 and 17 – though with a subsequent, three-month reflection period.
What changes were made?
The Bill was amended as it moved through Holyrood.
At stage two, it was announced 16 and 17-year-olds would need to live in their acquired gender for six months rather than three before applying for a GRC.
It was also announced there would be new statutory aggravation to the offence of making a fraudulent application for a GRC.
At stage three, MSPs backed a change tabled by SNP MSP Gillian Martin which means anyone subject to a sexual harm prevention order or sexual offences prevention order will not be allowed to seek a GRC.
Campaigners in favour of the Bill say a move to make trans people’s lives easier is long overdue.
A group of LGBT+ organisations issued a joint letter saying the Bill is a “historic opportunity to continue Scotland’s journey towards full social and legal equality”.
They disagree that an expansion of trans rights comes at the expense of women’s rights, saying the Bill will have little impact outside the trans community.
Nicola Sturgeon, whose government launched the Bill, said the current system of medical diagnoses is “intrusive, traumatic and dehumanising”.
What do opponents say?
Those in opposition to the Bill say it will impact the safety of women and single-sex spaces.
They say there are insufficient safeguards to protect women and girls from predatory men, raising concerns about environments like women’s prisons.
Protests inside and outside the Scottish Parliament called on MSPs to vote down the Bill.
Some oppose the Bill on religious grounds, saying it will blur the distinction between men and women.
Unusually in Scottish politics, opposition to the Bill has cut across party lines.
In October, the SNP’s Ash Regan dramatically quit the government in protest against the Bill, as nine of the party’s MSPs either abstained or voted against stage one of the legislation.
Others in the SNP, like leadership contender Kate Forbes, are also opposed to reforming the gender recognition process.
Most Conservative MSPs are opposed to the Bill, with exceptions being Jamie Greene and Sandesh Gulhane.
Scottish Labour is largely in favour, but national party leader Sir Keir Starmer has said he has concerns about some elements of it.
Scottish Green and Lib Dem MSPs are in favour of the new law.
Conservative ministers in the UK Government, including Kemi Badenoch, have argued it could impact the operation of equalities law around the UK.
This was the reasoning behind the decision to block the Bill with Section 35 of the Scotland Act – an unprecedented move which stops it gaining royal assent.
The SNP’s reaction to the use of Section 35 was delayed by Ms Sturgeon’s resignation and the subsequent leadership election, which saw Humza Yousaf elected as First Minister.
On the campaign trail, he said he would go to court to defend the Bill – but only if legal advice told him the case is winnable.
With Mr Yousaf seeking a judicial review at the Court of Session – Scotland’s highest civil court – judges will rule on whether the UK Government had “reasonable grounds” to invoke Section 35.
A ruling is likely to take several months and there could be appeals to courts in London.