Rishi Sunak would be showing "complete contempt" for the Scottish Parliament if he decides to block landmark gender reforms, Nicola Sturgeon has insisted.
The Scottish First Minister and SNP leader said it would be an "outrage" and evidence of the Tories using trans people as a "political weapon".
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 were 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 the Prime Minister is considering whether to trigger Section 35 of the Scotland Act - a blocking mechanism - to prevent the reforms.
The Act, which established a devolved Scottish government and Parliament, gives Westminster four weeks to consider bills passed by Holyrood that could have an "adverse effect on the operation of the law".
The power has never been used before and Mr Sunak has until Wednesday to decide whether to use what has been described as the "nuclear option".
Speaking at a press conference in Edinburgh on Monday, Ms Sturgeon said: "I think it would be an outrage.
"In my view there are no grounds to challenge this legislation. It is within the competence of the Scottish Parliament.
"It doesn't affect the operation of the Equality Act and it was passed by an overwhelming majority of the Scottish Parliament after very lengthy and very intense scrutiny by MSPs of all parties represented in the Parliament."
She added: "If there is a decision to challenge then in my view it will be quite simply a political decision. I think it will be using trans people - already one of the most vulnerable, stigmatised groups in our society - as a political weapon.
"I think that would be unconscionable, indefensible and quite disgraceful".
The Scottish First Minister also said a challenge would show "complete contempt" for Holyrood and principle of devolution.
The Prime Minister's official spokesman said today that "no decision has been taken at this point by the UK Government" - but would be done by Wednesday.
They added: "As you know it is standard and routine practice for us to consider any bill which the Scottish Parliament passes to understand any possible impact it may have on reserved legislation, both within Scotland and across the United Kingdom."
