The UK government denied Tuesday it was dismantling Scottish self-rule after exercising an unprecedented veto to block a new law passed already by the Edinburgh parliament.
The legislation would make it easier for people to self-identify as transgender in Scotland, without the need for a medical diagnosis, and lower the qualifying age from 18 to 16.
The standoff has ratcheted up London's tense relations with the Scottish government, less than two months after Nicola Sturgeon's Scottish National Party (SNP) saw its efforts to hold a new independence referendum stymied by the UK Supreme Court.
The Scottish parliament, set up in 1999, sets policy in Scotland in areas such as health and education, with the UK government in London retaining powers to determine issues affecting all four nations of the United Kingdom, such as defence and foreign affairs.
Interviewed on BBC TV, First Minister Sturgeon called the UK government's block a "direct attack on the institution of the Scottish parliament", and also an attack on a "stigmatised, vulnerable, often marginalised group".
She vowed to fight it in the courts. But in London the Conservative government's secretary of state for Scotland, Alister Jack, said the Scottish measure would have "a serious adverse impact" on UK-wide equality legislation.
"The government shares the concerns of many members of the public and civic society groups regarding the potential impact of the bill on women and girls," he told parliament.
"This is absolutely not about the United Kingdom government being able to veto Scottish Parliament legislation whenever it chooses, as some have implied," he added.
"The power can only be exercised on specific grounds, and the fact that this is the first time it has been necessary to exercise the power in almost 25 years of devolution emphasises that it is not a power to be used lightly."
Prime Minister Rishi Sunak's office denied the UK government was split on the emotive issue, after Education Secretary Gillian Keegan noted that when she was 16, she had already left school and was looking after herself.
Asked if 16 was too young to change gender, she told Sky News: "No I don't, actually.
"I was working at 16, I was paying tax at 16, I was making decisions for myself at 16.
"But it's not really about what I think, it's how we make sure we get that right balance of supporting children, but also making sure that what they're getting taught in schools is age-appropriate," Keegan added.
- 'Basic decency' -
Debate about trans rights has been turning increasingly acrimonious in Britain, with some women including "Harry Potter" author J.K. Rowling receiving online death threats for insisting on the equal rights of biological women.
But other countries including Belgium, Denmark and Ireland have already introduced gender self-identification, and the move is backed by World Health Organization guidance, according to the SNP.
Outside Sunak's Downing Street office in London, about a dozen people led by 25-year-old trans rights campaigner Owen Hurcum staged a vocal protest.
"It's quite frankly ridiculous that this transphobic Conservative government overturned devolution, democracy and basic decency to target a small group of people who just want even the semblance of something resembling dignity," Hurcum told AFP.
Sturgeon said Sunak's Conservatives were bent on "culture war" at the expense of transgender people, and rejected one argument that predatory men could exploit the legislation to gain access to women-only spaces.
The SNP has in turn exploited the row to build its case for Scottish independence from an overbearing UK.
But Sturgeon was forced to downplay signs of party discord over her declared intent to turn the UK's next general into a de-facto vote on separation.
After the Supreme Court blocked a second plebiscite without London's approval, senior voices in the SNP are openly unhappy with Sturgeon's tactics and fear the election gambit could backfire by stoking support for other parties.
"It's called internal democracy," Sturgeon said, insisting the SNP was having a "proper, full discussion" in the buildup to a special party conference on March 19.