Nicola Sturgeon has the warned the UK Government it would be an "outrage" if Tory ministers block gender recognition reforms passed by Holyrood.
The First Minister hit out after reports claimed Rishi Sunak will act on legal advice this week and halt the GRR legislation from becoming law.
It would be the first time that legislation passed by MSPs in the Scottish Parliament will have been struck down by Westminster since the devolution era began in 1999.
Speaking at a press conference today on the NHS, Sturgeon was asked if such a decision would be an outrage.
The SNP leader said: "Yes, 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 and it doesn't affect the operation of the Equalities Act.
"And it was passed by an overwhelming majority of the Scottish Parliament after lengthy and intense scrutiny by MSPs of all parties.
"So if there is a challenge to decision, in my view it will be quite simply a political decision and I think it will be using trans people - already one of the most vulnerable groups in our society - as a political weapon.
"I think it will be unconscionable and indefensible and really quite disgraceful."
Sturgeon added: "We have got Westminster politicians refusing to accept the right of the parliament to legislate on an area of its own competence, and making unwarranted and unjustified threats to challenge that legislation.
"If anyone is trying to use it politically, it's those Westminster politicians".
Downing Street said today that Scottish Secretary Alister Jack will make an announcement on whether to block the GRR bill.
The Prime Minister’s official spokesman said: "No decision has been taken at this point by the UK Government.
"It’s the Secretary of State for Scotland who is the ultimate decision-maker and you can expect to hear from him before the deadline on Wednesday."
MSPs last month passed the Gender Recognition Reform (Scotland) Bill by 86 votes to 39.
It allows trans people to obtain a gender recognition certificate (GRC) without the need for a medical diagnosis.
The bill 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.
The UK Government immediately raised concerns and claimed the GGR bill could impact on UK equality law and other pieces of UK-wide legislation.
Downing Street has also expressed concern about the impact of single-sex spaces and the checks and balances involved in the process of gaining a gender recognition certificate.
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