Rishi Sunak’s government has blocked legislation passed by the Scottish parliament that would make Scotland the first part of the UK to introduce a self-identification system for people who want to change gender. The Scottish secretary, Alister Jack, announced that he would use section 35 of the Scotland Act 1998 for the first time to halt the gender recognition bill after a review by UK government lawyers. Westminster’s decision to use the “nuclear option” of blocking the bill from going for royal assent represents a significant escalation of tensions around the issue, and will enrage supporters of the changes and nationalists. UK ministers, who met in Westminster on Monday to consider how to approach the legislation, are concerned the bill will have an “adverse impact” on UK-wide equalities law. Sources said blocking it would protect the devolution settlement and denied claims they were trying to inflame tensions as part of a culture war. Nevertheless, Nicola Sturgeon has said there were “no grounds” for the UK government to block the legislation, claiming that it did not affect the operation of the Equality Act. Scotland’s first minister has said her government was likely to mount a legal challenge in response, saying the use of section 35 would create a “very, very slippery slope indeed” and would embolden the UK government to do the same in other areas. After the announcement, she tweeted: “This is a full-frontal attack on our democratically elected Scottish parliament and it’s ability to make its own decisions on devolved matters. @scotgov will defend the legislation and stand up for Scotland’s parliament. If this Westminster veto succeeds, it will be first of many.” A court battle would inevitably be presented by the SNP as Westminster denying Holyrood its democratic right to make its own laws – hot on the heels of the supreme court verdict on another referendum – and could bolster the independence cause. The Scottish secretary, who will lay the order in the Commons on Tuesday, said: “After thorough and careful consideration of all the relevant advice and the policy implications, I am concerned that this legislation would have an adverse impact on the operation of Great Britain-wide equalities legislation. “Transgender people who are going through the process to change their legal sex deserve our respect, support and understanding. My decision today is about the legislation’s consequences for the operation of GB-wide equalities protections and other reserved matters. “I have not taken this decision lightly. The bill would have a significant impact on, amongst other things, GB-wide equalities matters in Scotland, England and Wales. I have concluded, therefore, that this is the necessary and correct course of action. “If the Scottish government chooses to bring an amended bill back for reconsideration in the Scottish parliament, I hope we can work together to find a constructive way forward that both respects devolution and the operation of UK parliament legislation.” The law, first proposed by Sturgeon six years ago, was passed by the Scottish parliament by 86 votes to 39, with the overwhelming support of the SNP, Labour, the Greens and the Lib Dems in December, after years of consultation and debate. During the bill process, which culminated in an unprecedented two days of late night sittings as MSPs worked cross-party to address concerns about abusive males potentially taking advantage of the new system, questions were also raised about how the new legislation would affect UK-wide equality law. This became a particular concern after a judgment from Scotland’s highest court, only a few weeks before the final vote, which ruled that – for the purposes of the 2010 Equality Act, the meaning of “sex” should include transgender women in possession of a gender recognition certificate. The legislation would make it easier for transgender people to obtain official gender recognition certificates, including by reducing waiting times, removing the need for a medical diagnosis and bringing the minimum age down from 18 to 16. The Labour leader, Keir Starmer, said on Monday that 16-year-olds should not legally be able to change gender, putting him at odds with his party in Scotland. The shadow Scottish secretary, Ian Murray, said the issue were “too important to be reduced to the usual constitutional fight”, and questioned why ministers at Westminster and Holyrood did not work together on an amended bill “to avoid this unnecessary stand-off”. Speaking to reporters earlier on Monday, Sturgeon accused Sunak’s government of “using trans people as a political weapon”. UK government sources, however, claimed that the legislation could have an adverse impact across the UK in areas like equal pay, single sex spaces and prison transfers. They believe that Sturgeon underestimated the degree of opposition to her bill. A YouGov poll for the Times after the law was passed claimed that two-thirds of Scots are opposed to its main features. Some Tories believe they can boost their vote in Scotland at the next election if they reduce politics to culture wars. The Scottish Greens, who made gender recognition reform one of their red lines for going into partnership with the SNP government after 2021 Holyrood election, said that it was “a dark day for devolution, democracy and trans rights”. Green MSP Maggie Chapman said: “Only one month ago, MSPs voted overwhelmingly for gender recognition reform. It was a proud day for equality, and for our parliament. To see a reactionary Tory government trying to block or overturn it is nothing short of outrageous, and we will resist them every step of the way.” Nancy Kelley, chief executive of Stonewall, said: “It is a matter of grave and profound regret that the prime minister has allowed trans people’s lives to be used as a political football. This is not governing with compassion.”