Rishi Sunak's Tory Government has formally blocked landmark gender reforms passed by the Scottish Parliament in a constitutional first.
The decision has led to a monumental row, with Nicola Sturgeon accusing ministers of a "direct attack" on Holyrood and a "profound mistake".
The Scottish First Minister has suggested the issue will inevitably end up in the courts - saying she will "vigorously defend" the Gender Recognition Bill.
Speaking in the Commons on Tuesday, the UK Government's Scotland Secretary Alister Jack defended the decision, but said it had not been taken "lightly".
Here we take a look at what the row is all about.
What does the legislation propose?
Passed overwhelmingly - 86 to 39 votes - by MSPs at Holyrood in December, the legislation is intended to make it easier for transgender people to legally change their gender.
It would remove the requirement for a medical diagnosis of gender dysphoria in order to obtain a Gender Recognition Certificate (GRC) - a legal document.
This was something proposed in 2017 by ex-Tory Prime Minister Theresa May before it was binned by her successor in No10 Boris Johnson and the then-equalities minister, Liz Truss.
The Scottish legislation would also allow 16 and 17-year-olds to apply for a GRC for the first time, and would reduce the amount of time a person has to live in their acquired gender before they can be granted the document.
What has the UK Government done?
In a statement on Monday, Scottish Secretary Alister Jack announced he would trigger section 35 of the Scotland Act - blocking the legislation.
It is the only time the power - described as the "nuclear option" - has been used by the UK Government since the Scottish Parliament was created in 1999.
Mr Jack said it would prevent the legislation from proceeding to Royal Assent and therefore prevent it becoming law.
He claimed not to have taken the decision while No10 suggested it was "forced to to take this unprecedented approach".
Why did ministers trigger the 'nuclear option'?
The Scottish Secretary argued on Tuesday the Gender Recognition Reform (Scotland) Bill would have "serious adverse effects" on the operation of UK-wide equalities legislation.
Giving a statement to MPs, Mr Jack said "very senior legal opinion" is that using the Section 35 order to block the reforms was "appropriate".
He had already claimed the reforms - which simplify the process for trans people to obtain a gender recognition certificate (GRC) - could have "chilling effects on single-sex spaces" for women.
Ms Sturgeon, however, has insisted exemptions around single-sex spaces will be unaffected by the legislation.
In a 'statement of reasons' the UK Equalities Office also set out three reasons behind the decision, including a claim there would be impact on single-sex clubs or associations and equal pay comparisons.
What has been the response?
The move has led to a major bust-up with the Scottish First Minister and SNP leader alongside members of Holyrood.
Ms Sturgeon accused ministers of a "full-frontal attack" on the Scottish Parliament and "its ability to make its own decisions on devolved matters''.
Scotland's Justice Secretary Shona Robison also described it as a "dark day for trans rights and a dark day for democracy in the UK".
The Chief Executive of the LGBT+ charity Stonewall also accused the Government of using trans people's lives as "political football".
She described the decision to block the legislation as the "nuclear option", adding: "This is not governing with compassion".
She added: "These are not the actions of a government that can stand on the international stage as a credible defender of LGBTQ+ rights.
But Tory MPs - and some Labour MPs - defended the decision during a Commons debate on Tuesday.
What will happen next?
The most likely route is the courts - through a judicial review.
In an interview with the BBC on Tuesday the Scottish First Minister said the decision will "inevitably" end with a legal challenge.
She said it will not automatically go to the Supreme Court, suggesting the Scottish Government's first course of action would be a judicial review.
She added: "Now, obviously, we will see what reasons the UK Government managed to set out in the order they present before the House of Commons.
"But I can say categorically, the Scottish Government will vigorously defend this legislation."
Speaking in the Commons, Mr Jack also appeared to concede the row will progress to the courts.
