The UK Government could block controversial plans to make it easier for Scots to change their gender despite MSPs voting in favour of the landmark legislation.
The Gender Recognition Reform (GRR) Bill was finally passed at Holyrood today by 86 votes to 39 after winning cross-party support following three days of heated debate.
But nine SNP MSPs voted against in the party's biggest rebellion since taking power in 2007.
The legislation intends to lower the age from 18 to 16 at which a person can apply to change their legal gender, as well as removing the need for a medical diagnosis.
But the plans could be scuppered after Tory ministers hinted they could step in to stop the act receiving Royal Assent.
Alister Jack, the Scottish Secretary, said: "We share the concerns that many people have regarding certain aspects of this Bill, and in particular the safety issues for women and children.
"We will look closely at that, and also the ramifications for the 2010 Equality Act and other UK wide legislation, in the coming weeks - up to and including a Section 35 order stopping the Bill going for Royal Assent if necessary.”
A section 35 order is intended to prevent laws passed by the Scottish Parliament having "an adverse effect on the operation of the law as it applies to reserved matters".
Kemi Badenoch, the UK equalities minister, previously raised concerns the GRR bill would impact other home nations as well as Scotland.
The new law was passed despite nine SNP MSPs rebelling against the Scottish Government. Two Labour MSPs also defied their party to vote against it, while three Tories supported the bill.
Nicola Sturgeon had earlier defended the legislation and described it as possibly the most scrutinised in the history of Holyrood.
The First Minister rejected Tory claims that it failed to offer adequate safeguards to protect women-only spaces.
She said: "I will never apologise for trying to spread equality, not reduce it, in our country."
Parliamentary proceedings were temporarily halted on several occasions as members of the public heckled MSPs before the final vote was taken.
Cries of "shame on you" could be heard after it was announced the legislation had passed with cross-party support.
But supporters of reform led a loud round of applause.
Shona Robison, the social justice secretary, described it as an "historic day" for Scotland.
She also insisted the new legislation did not interfere with equalities law reserved to Westminster.
"This is an historic day for equality in Scotland with the Gender Recognition Reform Bill being approved by parliament and by members of all parties," she said.
"It simplifies and improves the process for a trans person to obtain a gender recognition certificate – which many currently find intrusive, medicalised and bureaucratic.
"The legislation makes no change to the reserved Equality Act 2010 and that principle is enshrined in the Bill. As I have made clear, the Scottish Government continues to support the provision of single-sex services and the rights of women.
"The passing of this bill is a significant step forward in creating a more equal Scotland, where trans people feel valued, included and empowered."
Labour's Pam Duncan-Glancy said the legislation provides MSPs with "one of those rare moments… where we all have a real opportunity to improve lives and directly tackle inequality".
She said the Bill will help “society to accept them (trans people) and to support them to be their best selves, without barriers or additional costs or medicalisation”.
She insisted: "I believe strongly the reform we will vote for today has been a long time coming, and that is why changing the current onerous, lengthy and invasive process of legal gender recognition has always been so important to me.”
MSPs spent 24 hours this week debating it after two marathon sessions considering amendments on Tuesday and Wednesday – with the latter finishing at 1.15am on Thursday.
The Tories have been accused of an attempt to filibuster the legislation, proposing numerous points of order and forcing almost all amendments to votes that were sometimes not required.
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