Scotland's First Minister Nicola Sturgeon suggested the bill would strengthen the case for Scottish independence
Scotland and Wales said the British government's internal market bill, which a minister admitted breaks international law over Brexit, will undermine the United Kingdom by stealing powers from Wales, Scotland and Northern Ireland.
The bill, a blueprint for life outside the European Union, will be published on Wednesday. A government minister has acknowledged it would break international law in a "limited way", and it jeopardizes trade talks with the EU.
Under the United Kingdom's delicate constitutional balance, semi-autonomous parliaments and governments in Scotland, Wales and Northern Ireland known as the devolved administrations have powers over areas like education, health, policing and justice.
Scotland's First Minister Nicola Sturgeon, leader of the pro-independence Scottish National Party (SNP), described the UK government's new bill as "a full frontal assault on devolution" that would steal powers from Edinburgh, Cardiff and Belfast.
She suggested the bill would strengthen the case for Scottish independence.
"More and more this is not about independence v the status quo of devolution. It's about independence as the only way to protect the Scottish Parliament from being undermined and its powers eroded," she said.
Sturgeon described Prime Minister Boris Johnson's government in London as "the most reckless (and to make it worse, incompetently so) and unprincipled in my lifetime".
Reaction to the bill was equally negative in Cardiff, where it was described as a threat to the ties binding the four parts of the United Kingdom together.
"Let me be clear – the UK government plans to sacrifice the future of the union by stealing powers from devolved administrations," said Jeremy Miles, Wales' counsel general and minister for European transition.
"This bill is an attack on democracy and an affront to the people of Wales, Scotland and Northern Ireland, who have voted in favour of devolution on numerous occasions."