Nicola Sturgeon has said the UK government would be using trans people as a “political weapon” if it decides to block Scottish gender reforms.
The first minister said any veto of the legislation would be an “outrage”.
The UK government said it had not yet decided whether to use powers which would stop the bill becoming law.
Ms Sturgeon also criticised Labour leader Sir Keir Starmer for saying that 16-year olds were too young to decide about changing their gender.
Holyrood MSPs backed a move towards a self-identification system last month.
It would mean that people in Scotland would no longer require a medical diagnosis to change gender. The timescale involved would also be reduced.
The UK government is considering using its own powers to block the Scottish legislation.
Under Section 35 of the Scotland Act, ministers can stop a bill getting royal assent – a power which has never previously been used.
Downing Street said Scottish Secretary Alister Jack would make an announcement ahead of Wednesday’s deadline.
The prime minister’s 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.”
Nicola Sturgeon said there were no grounds to challenge the legislation, as it falls within the competence of the Scottish Parliament.
She said: “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 intense scrutiny by MSPs of all parties.
“If there is a decision to challenge, then in my view then it will quite simply be a political decision and it will be using trans people, already one of the most vulnerable stigmatised groups in our society, as a political weapon.
“I think that will be unconscionable, indefensible and really quite disgraceful.”
UK ministers have concerns the Scottish system could come into conflict with UK-wide equalities law.
Prime Minister Rishi Sunak has said the government was taking advice on the implications of the reforms “as is completely standard practice”.
Ms Sturgeon said the significance the UK government using Section 35 powers would “go beyond the particular subject matter” of gender reforms.
She added: “There is a bigger issue of principle here, the right of the Scottish Parliament to legislate within its areas of competence.
“If we see a challenge this week then we will be seeing yet more evidence from this UK government of complete contempt for the Scottish Parliament and devolution.”
The first minister also said it would “embolden” the UK government to use Section 35 powers on other issues, which she described as “a very slippery slope indeed”.
At a Scottish government media briefing on the health service, Ms Sturgeon was also asked about Sir Keir Starmer’s views on the gender reforms.
The Labour leader told the BBC’s Sunday with Laura Kuenssberg he had concerns and felt 16-year-olds were too young to change their legally recognised gender.
But he stopped short of supporting a challenge to the legislation.
Ms Sturgeon pointed out that Scottish Labour had voted for the legislation at Holyrood.
She said: “If he backed any move by the government to block this he would be showing utter contempt for his own Scottish party as well as the Scottish Parliament.”
A survey conducted for the BBC last year showed general sympathy towards trans people alongside uncertainty over some of the details of Scotland’s gender reforms.
The controversial legislation, which is expected to come into force later this year, lowers the age at which people can apply for a gender recognition certificate from 18 to 16.
It also removes the need for a medical diagnosis of gender dysphoria, while applicants only needing to have lived as their acquired gender for three months rather than two years – or six months if they are aged 16 or 17.
The BBC poll found people aged 16 to 34 were more likely than older people to support making it easier to acquire a gender recognition certificate and also backed allowing people to legally identify as non-binary.
In addition to the generation split, it found there was also a tendency for women to be more supportive than men.
Meanwhile, last night (Monday) Scottish Trans, the trans equality project of LGBTI equality and human rights charity the Equality Network, condemned the UK Government’s announcement that they will seek to block the Gender Recognition Reform (Scotland) Bill
Vic Valentine, Manager of Scottish Trans, said: “The bill as passed would introduce a simpler and fairer way for trans men and women to be legally recognised as who they truly are, allowing them to live with the dignity we all deserve.
“It was passed by the Scottish Parliament by 86 votes to 39, with the overwhelming support of the SNP, Labour, the Greens and the LibDems. That followed years of consultation, and lengthy Parliamentary consideration and debate. The bill covers matters that are devolved to the Scottish Parliament, and its consequences were considered by MSPs in great detail.
“For the UK Government to seek to block the Scottish democratic process in this way, simply because they disagree with the welcome decision the Scottish Parliament has made to improve trans people’s lives, is unacceptable. We fully expect the Scottish Government to challenge this in the courts.”
All the parties in the Scottish Parliament except the Conservatives committed to improve gender recognition law in their 2016 and 2021 Holyrood manifestos. The Scottish Government consulted publicly on the proposals in 2017, then published the bill and consulted again in 2019.
The Parliament’s Equalities, Human Rights and Civil Justice Committee consulted again in the spring and summer of 2022, and heard oral evidence on the bill from 39 witnesses. Those included supporters and opponents of the legislation, and experts in matters raised about it, such as equality law, women’s services, young people, sport and prisons, and religious representatives. They also heard from the leader of the Irish Seanad, about experience with similar legislation passed there some years ago.
The Committee recommended that the Parliament approve the principles of the bill.
Over 300 amendments were then considered at stages 2 and 3 of the bill, and more than 80 of them, from MSPs of different parties, were agreed upon to address matters that had been raised in relation to the bill.