The stern warning was issued following a review by Cardiff of the Conservative government’s white paper outlining plans for the UK internal market.
The document released last month was denounced at the time as a ‘takeover’ by devolved administrations in Scotland and Wales.
While the Scottish National Independence Party holds power in Edinburgh, there is a pro-union Labor-led administration in Cardiff.
In a letter to Alok Sharma, Britain’s Secretary for Business, Welsh Brexit Minister Jeremy Miles accused the Westminster government of undermining established standards of consultation with devolved administrations.
‘The Welsh government fears that the UK’s long-term survival will be strained and that the approach taken in the White Paper will exacerbate these tensions in a way which, if not resolved, will accelerate the break-up of the Union, ”said Miles.
Presenting the white paper to the UK parliament last month, Mr Sharma said the legislation would give ‘regulatory clarity and certainty’ to UK businesses after the end of the Brexit transition period, when the UK will no longer follow thousands of rules and standards established in Brussels.
Among the most controversial proposals in the document was a program to force ‘mutual recognition’ of products across all parts of the UK, effectively allowing Mr Johnson’s government to unilaterally set food and environmental standards for the country. of Wales and Scotland.
Mr Miles said the Welsh administration’s initial view that the document was ‘fundamentally flawed and misleading’ was confirmed by subsequent detailed analysis.
He added that the approach proposed by the Westminster government, if followed, “would represent a direct attack on the current model of deconcentration” and “emasculate the current rights of deconcentrated institutions to implement changes in the environment. regulatory ”.
He concluded that the Westminster legislation envisioned in the White Paper was “unnecessary, impractical [and] authoritarian ”and would not obtain the consent of the Welsh parliament.
Mr Miles pointed out how legislation could sever the wings of the Cardiff parliament in several areas. He noted how the Welsh administration decided to mirror an EU directive banning nine single-use plastics, while in England only three were banned.
“The principle of mutual recognition [envisaged in the white paper] could mean that Wales would not be able to introduce legislation or, if legislation is introduced, enforce the ban on the sale of these six [single-use plastic] articles in Wales, regardless of their origin, ”states the analysis of the white paper attached to Mr Miles’ letter.
A government spokesperson said: ‘Our proposals ensure that businesses can continue to thrive after the end of the transition period by ensuring their ability to trade freely with every nation in the UK, as they have done for centuries. . According to our plans, decentralized administrations will have power over more matters than they have ever had before, and will continue to have the power to regulate within their nations.
“The UK is a world leader in agricultural, environmental and food standards and that will not change. At the end of the transition period, all these existing standards will be kept in our domestic law and we will not sign any trade agreement that would compromise our high standards, ”they added.
Other potential areas where the powers of decentralized administrations could be reduced in the future include building regulations, educational qualifications and food standards, the analysis added.
Daniel Wincott, professor of law and society at Cardiff University, said Mr Miles’ unusually strong language should give the Westminster government pause for thought.
“The Johnson administration has learned to sidestep the robust language of the Scottish government,” he said. “When their proposals on the internal market are also rejected in a blanket way by Cardiff, it may be time for the UK government to reconsider.”
The Welsh administration’s concerns over the White Paper echo those raised in Scotland, where Scottish Secretary of the Constitution Mike Russell last week accused the UK government of a ‘scandalous takeover’.
Giving evidence to the Scottish Parliament’s finance and constitution committee, Mr Russell said his government was considering legal action to challenge the plans, which he called a ‘major weakening of decentralization’.
“My opinion is that it completely reveals the agenda of the current UK government, which is hostile to decentralization and hostile in particular to Wales and Scotland for exercising their rights under decentralization,” said he added.
Growing constitutional tensions stemming from the implementation of Brexit could help the SNP as it seeks to retain power in Edinburgh in next May’s election.
Recent opinion polls suggest the SNP is on track to victory and that a majority of Scottish voters already support the UK’s departure. Scotland voted against Brexit in the 2016 EU referendum.
The UK Trade Department did not immediately respond to a request for comment.