For the first time in the 24-year history of the Scottish parliament, Westminster has overruled a decision made in Edinburgh in a row over transgender rights.
Scotland's first minister Nicola Sturgeon has accused London of launching a "direct attack on the institution of the Scottish Parliament" after Westminster blocked legislation on transgender rights.
The Scottish parliament has been sitting at Holyrood since 1999 and has devolved powers.
In a new precedent, the UK government used Section 35 powers to overrule legislation passed in Edinburgh aimed at making it easier for people to self-identify as transgender without the need for a medical diagnosis. London says the legislation could have an impact on equality laws that apply across Great Britain.
In Westminster, Alister Jack, Secretary of State for Scotland, addressed MPs.
He said: "It is our assessment that the bill would have a serious adverse impact, among other things, on the operation of the Equality Act 2010. Those adverse effects include impacts on the operation of single-sex clubs, associations and schools, and protections such as equal pay."
Mrs Sturgeon says the issue will "inevitably end up in the Supreme court".
Demonstrators outside Westminster accused parliament of transphobia.
"They're lying when they say it's about anything other than transphobia,” said trans rights campaigner Owen Harcum. “There is nothing about that act that will overrule or damage the Equality Act as it stands in the UK."
