The UK government has announced it will use its powers over the Scottish parliament to veto a law that would make it easier for transgender individuals to alter their legally recognised sex.
Its Scottish secretary has argued the bill comes into conflict with an existing equality protection law, which applies UK-wide.
The move comes after weeks of tensions between the two parliaments during the bill's debate.
It is the first time the UK government has stopped Scottish legislation after it had been voted in.
What's the new law the UK government has a problem with?
The new law — passed by the Scottish parliament late in December — allows transgender individuals over the age of 16 years to apply for a birth certificate declaring their legal gender without a medical diagnosis of gender dysphoria.
Despite significant dissent — which saw the resignation of one minister — the Scottish parliament passed the legislation with cross-party support six years after it was proposed by First Minister Nicola Sturgeon.
The legislation rests on the model of self-identification: that an individual should be able to declare themselves transgender without jumping through excessive legal or medical hoops.
Under the law, an individual would need to prove they have lived as a certain gender for a period of at least three months, or six months if they are under the age of 18 years.
There will also be a three-month "reflection period" during which they can change their minds.
Comparatively, Tasmania's laws regarding gender recognition are the most liberal in Australia and require only a statutory declaration from the individual, who must be at least 16 years of age.
Previously, transgender individuals in Scotland had to be over the age of 18 years to declare themselves a different gender to what they were assigned at birth, needed to have lived as that gender for two years, and have received a medical diagnosis of gender dysphoria.
Why is UK government determined to block the law?
Tensions between the Scottish and British parliaments have been sky-high in recent weeks over concerns for the law.
Primarily, the Rishi Sunak government says the gender laws are incompatible with the existing Equalities Act, which applies UK-wide.
It argues that different criteria for a transgender person to be legally recognised between Scotland and England would endanger same-sex spaces in the latter.
Critics have cited concerns over the protections of single-sex schools, scholarships and grants, as well as safety in women's bathrooms, shelters and prisons among other settings.
The Scottish government says the law would have no impact on the Equality Act, and says the new system would not grant any additional rights that would threaten those institutions.
However, the UK government has voiced concerns over "gender tourism", in which people in England might use the new Scottish processes to change their legal gender more easily.
Under a reciprocal arrangement, England automatically recognises legal documents with chosen genders from a number of allied countries, including Scotland and Australia, however this is currently under review.
What power is it that allows UK to veto Scottish law?
Scotland Secretary in Westminster Alister Jack announced he would invoke Section 35 of the 1998 Scotland Act.
That move prohibits Scotland's presiding officer from submitting a bill for Royal assent, therefore preventing it becoming law.
The Scotland Act is the law that created Scotland's parliament after a 1997 referendum.
For many hundreds of years prior, Scotland did not have a government following the unification of Great Britain in the 1700s.
The Act allows the Scottish parliament to make decisions on domestic issues without needing approval from the UK parliament.
This is the first time the UK government will exercise its veto powers over the Scottish government.
What happens next?
Scotland has vowed to fight the veto on the grounds that, if allowed to proceed, Westminster may become emboldened to veto further Scottish legislation.
This would involve legal action in the courts, but precisely how the showdown will proceed remains to be seen.
It's also unclear how the independence movement in Scotland will be fuelled by this.
At present, the Scottish government is divided on the issue, however unwelcome interference from the UK government could sway opinion.
Scotland is not permitted to hold another vote on independence after a 2014 referendum resulted in a narrow vote against independence.
