The UK government decided to block a Scottish law on changing gender on Monday, risking a standoff between London and the devolved government in Edinburgh.
The law -- passed by a wide margin in December with support from all five political parties in the Holyrood parliament -- makes it easier for people to legally change their gender, but is now opposed by London over concerns about its possible impact on UK-wide equality laws.
This is the first time London has used its legal powers to block a law in Scotland.
Under the Scotland Act of 1998, a devolved government was created in Scotland, which gave the country a greater say over how it is run. But many areas of law and policy remain under London's control, which is the supreme authority.
Monday's move risks sparking another political and legal battle between London and Edinburgh, just two months after the Scottish parliament was prevented by the UK Supreme Court from holding a second independence referendum.
Secretary of State Alister Jack, the UK government's representative in Scotland, said on Monday evening that the gender bill would "impact" how equality legislation functioned across Britain.
"My ruling today is about the impact of the legislation on the operation of equality protections," he said. "I did not make this decision lightly."
Writing on Twitter, the Scottish First Minister Nicola Sturgeon denounced a "frontal attack on our democratically elected Scottish Parliament and its ability to make its own decisions".
She said the Scottish government would "defend" the law, slamming the "Westminster veto".
The law, proposed by the Scottish parliament in December, aimed to simplify the legal process for changing gender.
It removed the need for transgender people to have a medical diagnosis for gender dysphoria when applying for a gender recognition certificate (GRC).
The period that an applicant must live in their new gender was also reduced from two years to three months, while lowering the age that people can apply for a GRC from 18 to 16 -- which brings it into line with other rights for young people in Scotland, including voting.
Stonewall, a UK-based charity for LGBT+ rights, criticised London's decision, saying it was "saddened".
"There are not the actions of a government that can stand on the international stage as a defender of LGBTQ+ rights," it said in a statement.
The UK government warned it could block the law, immediately after it was passed.
Adopted after heated debates in the local parliament, opponents claim it will endanger women, affecting access to spaces reserved for them.
The Scottish government insists that the bill does not affect British law which authorises the exclusion of transgender people from certain reserved spaces, such as changing rooms for example.