Is Scotland's Gender Recognition Reform Bill opening a constitutional crisis?
There can be few in Scotland who haven't heard of the Gender Recognition Reform (GRR) Bill. Now the devolved legislation looks to become a wedge issue that will trigger a constitutional battle.
The Scottish Government legislation was passed by MSPs with cross-party backing before Christmas after years of consultation and debate.
It makes it quicker and easier for individuals to gain legal recognition of their transition from their birth sex to an acquired gender, cutting the waiting time from two years to three months and removing the need for a medical diagnosis of gender dysphoria.
They are changes aimed at introducing more dignity and respect to the process for transgender people.
And the shift also requires applicants to take a solemn oath to live out their lives in their acquired gender, introducing a new criminal offence of false declaration.
But the passage of the legislation has not removed concerns from critics of the bill that it is ill-defined and its impacts on the UK-wide Equality Act have not been fully considered. That's something Labour's Keir Starmer has raised concerns about.
Now we're waiting to find out if the UK Government will allow the Scottish bill to pass into law.
What it decides will have major political consequences.
How could this happen?
Scotland Secretary Alister Jack (above) raised the possibility of a Westminster challenge as soon as the bill was passed in late December, saying: "We share the concerns that many people have regarding certain aspects of this bill, and in particular the safety issues for women and children.
"We will look closely at that, and also the ramifications for the 2010 Equality Act and other UK-wide legislation in the coming weeks - up to and including a Section 35 order stopping the bill going for royal assent if necessary."
The order he refers to is part of the Scotland Act and gives the Secretary of State the "power to intervene in certain cases".
If used, it would allow Jack, in this role, to order the Presiding Officer of the Scottish Parliament not to send the GRR bill to the King for royal assent, therefore preventing it from becoming an Act of Parliament and becoming law.
When has this ever happened before? The answer to that is never - and so if Jack does so now, he is making political history and pushing our politics into untested waters.
How would this work?
To do so, the UK Government will have to decide that the Holyrood bill makes changes to the law "as it applies to reserved matters" which it believes "would have an adverse effect on the operation of the law as it applies to reserved matters".
And that's where the Equality Act 2010 comes in. Set in Westminster and applied across the UK, it is legislation that the Scottish Parliament does not have the legal powers to change.
The Scottish Government has said the GRR Bill is competent and does not create such a circumstance.
However, the signs are that this will be more likely than not.
And if one is to arise, it will do so soon - because there is a four week time limit following the passage of a bill in which an intervention can be made.
But that's not the UK Government's only option, because it could also seek to use its authority under Section 33 of the Scotland Act to refer the matter directly to the Supreme Court - which has only just finished dealing with the Scottish Government's failed bid to hold another indyref without Downing Street approval.
If it goes to the court, UK Government lawyers will seek to have the justices there determine that the bill relates to reserved matters and therefore oversteps Holyrood competencies.
We've seen that happen twice before, when laws agreed by MSPs over Brexit and the incorporation of the UN Convention of the Rights of the Child were challenged successfully.
The political ramifications of a challenge are now a major talking point. Former Scottish Labour leader Kezia Dugdale has said blocking the GRR bill's enactment "will anger nationalists and devolutionists" , while former Conservative MSP Adam Tomkins has said that "no parliamentary majority can make an unlawful enactment lawful, if unlawful it is".
"Blocking it should not 'anger devolutionists'," he said, "given that the power to block laws that interfere with reserved powers is baked into the devolution settlement. It would be upholding devolution to use s35."
Maggie Chapman of the Scottish Greens said a challenge "would be a shocking attack on trans rights and democracy" and would "totally undermine devolution and all of Downing Street's rhetoric and claims about us supposedly being better together and part of a union of equals".
For her part, First Minister Nicola Sturgeon said there are "no grounds" for challenge and her administration will "vigorously defend the legislation".
"If there is a decision to challenge, then in my view it will quite simply be a political decision and it will be using trans people, already one of the most vulnerable stigmatised groups in our society, as a political weapon," she said.