Downing Street has not ruled out mounting a court challenge to a law set to be passed later this month by the Scottish Parliament, which will simplify the legal process for anyone in Scotland who wants to change their gender.
The Scottish bill will shorten the timescale for anyone who wants to obtain a gender recognition certificate, a document allowing someone to change their gender on their birth certificate, from two years to three months.
The certificate means people can change some legal documents, and can also affect areas such as entitlement to benefits and pensions.
The Westminster government is also considering refusing to recognise certificates issued under the new Scottish system, which could cause problems for people with them who want to move to other parts of the UK.
Since 1999 some laws which apply in Scotland are made by MSPs in Edinburgh, while others are made by MPs at Westminster.
UK government ministers are responsible for gender laws in England and Wales, and have no plans to move in the same direction as the Scottish government.
The prime minister's official spokesman said: "We've made no decisions on any potential action at this time.
"As the Equalities and Human Rights Commission and UN special rapporteur have set out, the Scottish government's proposals currently raise a number of clear concerns."
In November, the UN rapporteur on violence against women and girls Reem Alsalem warned the legislation could increase risks to their safety by potentially allowing "violent males" to "abuse" the system.
Scottish ministers say there is "no evidence" women or girls will be harmed by the bill.
The Downing Street spokesman said: "In order to understand the potential impact of the bill across the United Kingdom we will obviously monitor its progress closely as we do with any potential Bill."
Asked if the UK government could challenge the legislation in the courts, he replied: "We've made no decisions at this time."
Scottish ministers have said they are happy to meet their UK counterparts to discuss their concerns – and officials in Edinburgh said they tried to set up talks in October, without any response.
The SNP-led government believes the bill encompasses powers held solely in Edinburgh, so the UK government would not have grounds to mount a legal challenge.
Equalities Secretary Kemi Badenoch has now written to the Scottish government expressing concerns about the legislation, and has offered to meet Scottish ministers to discuss it.
Scottish Social Justice Secretary Shona Robison has said she would be "happy to meet" Ms Badenoch.
A UK government source told the BBC they had concerns people from elsewhere in the UK might relocate to Scotland to change gender.
They claimed Scottish ministers wanted to paint Scotland as a "haven of inclusivity" in comparison to a "nasty Westminster".
The source described the Scottish legislation as a "test case scenario" of how a bill passed by the Scottish Parliament could "undermine Westminster competencies" – in other words, handing Holyrood powers beyond Scotland.
A separate UK official told the BBC there was "genuine concern" about the impact the legislation could have across the rest of the UK. They added that they hoped a legal battle could be avoided.
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