Rishi Sunak has set the stage for a major constitutional row after blocking the Scottish Parliament's landmark gender reforms.
Triggering the "nuclear option", the Scottish Secretary Alister Jack said he would use section 35 of the Scotland Act to stop the legislation.
It is the first time UK ministers have used the mechanism and comes despite warnings it would treat the Scottish Parliament with "contempt".
On Monday evening the Scottish First Minister Nicola Sturgeon described the move as a "full-frontal attack on our democratically elected Scottish Parliament".
The SNP leader warned if the "veto succeeds, it will be the first of many".
In December Holyrood overwhelmingly approved the Gender Recognition Bill - making it easier for trans people to legally change gender.
The legislation removes the requirement for a medical diagnosis of gender dysphoria in order to obtain a gender recognition certificate.
Similar laws were proposed by ex-PM Theresa May in 2017 but shelved by her successor Boris Johnson and his then equalities minister, Liz Truss.
The reforms will 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.
But in a statement on Monday, Mr Jack said: “I have decided to make an order under section 35 of the Scotland Act 1998, preventing the Scottish Parliament’s Gender Recognition Reform (Scotland) Bill from proceeding to Royal Assent.
“After thorough and careful consideration of all the relevant advice and the policy implications, I am concerned that this legislation would have an adverse impact on the operation of Great Britain-wide equalities legislation."
The UK minister added: “Transgender people who are going through the process to change their legal sex deserve our respect, support and understanding. My decision today is about the legislation’s consequences for the operation of GB-wide equalities protections and other reserved matters.
“I have not taken this decision lightly. The Bill would have a significant impact on, amongst other things, GB-wide equalities matters in Scotland, England and Wales. I have concluded, therefore, that this is the necessary and correct course of action.
At a press conference earlier, Ms Sturgeon also warned the Government not to treat trans people as a "political weapon".
"I think that would be unconscionable, indefensible and quite disgraceful," she said.
