The Scottish Government has vowed to "vigorously contest" any legal challenge from Westminster over its gender reform plans.
Shona Robison, the social justice secretary, flatly rejected claims from the UK Government that the Gender Recognition Reform (GRR) Bill could negatively impact UK-wide equalities laws.
MSPs from all parties voted to pass the controversial legislation by 86 votes to 39 on Thursday after three days of intense debate and protests outside Holyrood.
The passing of the Bill will remove the need for a medical diagnosis of gender dysphoria to obtain a gender recognition certificate (GRC).
Alister Jack, the Scottish Secretary, later warned the UK Government could block the legislation by issuing a rare section 35 order.
Mercedes Villalba, a Labour MSP, said: "Let's call this what it is, an attack on devolution."
UK equalities minister Kemi Badenoch also said the UK Government was “now looking at provisions that can prompt reconsideration” of the legislation.
The Tory MP said last night: "The SNP passed their gender recognition bill, despite strong opposition from even within their own party, the Equality and Human Rights Commission, the UN Special Rapporteur on Violence against Women and Girls and a variety of civic groups in Scotland.
"I share their concerns. Particularly, on this Bill's impact on the functioning of the Equality Act, which is designed to protect all UK citizens.
"The Scottish Government has not addressed the full implications of their bill - especially on the lives of women and girls. The UK Government is now looking at provisions that can prompt reconsideration and allow MSPs to address these issues."
But Robison promised today to defend the gender reforms.
"The Bill as passed is absolutely within legislative competence and of course was backed by an overwhelming majority with support from all parties,” she said on BBC Radio Scotland.
"I think any attempt by the UK Government to undermine what is, after all, the democratic will of the Scottish Parliament, it will be vigorously contested by the Scottish Government.”
Opponents of the legislation fear that it will impact on the Equality Act, particularly in relation to the exemption which allow for trans people to be excluded from single sex spaces in some circumstances.
The Bill, the Scottish Government says, will have no impact on the exemptions.
The legislation will make it easier for trans people to obtain a gender recognition certificate (GRC) by removing the requirement for a diagnosis of gender dysphoria.
It also lowers the minimum age for applicants to 16 and drops the time required for an applicant to live in their acquired gender from two years to three months – six for those aged 16 and 17 – though with a three-month reflection period.
MSPs embarked on an intense debate on the Bill this week, spending more than 24 hours on the consideration of amendments and the final vote.
Maggie Chapman, Scottish Greens equalities spokeswoman, said: "Time and again the UK government has shown total contempt for the LGBTQIA+ community. It has knowingly fanned the flames of prejudice and spread the most vicious smears and disinformation.
"Any attempt to block this Bill would be an attack on devolution and democracy and on the rights of trans people. The UK government rightly recognises gender recognition certificates from other countries - they should honour those issued in Scotland.
"Gender Recognition Reform is a small change that will make a big difference. It puts Scotland closer to following international best practice and will improve the lives of our trans siblings, who have experienced years of prejudice and discrimination."
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