Rishi Sunak is preparing to block laws in Scotland that make it easier for people to change gender, Sky News understands.
The move would be an unprecedented intervention in 25 years of devolution and is likely to spark a new constitutional row with Holyrood.
The Gender Recognition Reform (Scotland) Bill was passed by a majority of MSPs last month, with First Minister Nicola Sturgeon hailing at as a “historic day for equality”.
The new legislation will lower the age people can apply to change their gender to 16, remove the need for a medical diagnosis of gender dysphoria for a gender recognition certificate (GRC), and reduce the time an applicant needs to live in their acquired gender.
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Scotland’s gender bill explained
Why does the UK government want to block it?
While gender recognition is a devolved matter, ministers are concerned about the impact of the bill on equality laws, which are the preserve of Westminster.
Although the bill does not apply to England and Wales, the government fears it may lead to gender tourism and allow biologically male Scottish inmates in English jails to demand to be put in women’s prisons.
There is also concern that people who change gender in Scotland would have a different legal sex when they are in the rest of the UK, and that organisations offering single-sex spaces would have to adopt different policies.
Any action to block the legislation is likely to spark a furious response from the SNP, which has already accused Westminster of denying democracy by refusing to allow another independence referendum.
The planned intervention – first reported in The Times – comes after the government sought legal advice to stop the bill from getting royal assent next week.
The advice states that the legislation will have an adverse impact on UK-wide equality legislation, enabling ministers to block it.
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Mr Sunak has said it is “completely reasonable” for the UK government to consider blocking the reforms.
Scotland Secretary Alister Jack went as far as to say he may invoke section 35 of the Scotland Act, which gives him an effective veto on laws he believes impact on reserved matters.
The power has never been used before and in response to the threats, the Scottish government said: “Any attempt by the UK government to undermine the democratic will of the Scottish parliament will be vigorously contested.”
The challenge comes despite Mr Sunak saying he wants to work “constructively” with the Scottish government during a visit to Inverness on Thursday.
Following talks with Ms Sturgeon, the PM said that while they are “not going to agree on everything”, he believes there is scope for co-operation.