The United Kingdom could be broken up by the Conservatives and Welsh and Scottish nationalists, Wales and Northern Ireland Secretary Peter Hain has warned.
He said the UK was under threat from "an opportunistic coalition of myopic Tories and the narrow separatists of Plaid Cymru and the Scottish Nationalists".
Writing in the Western Mail, he accused the Tories of risking the "Balkanisation of Britain" by proposing to stop Welsh and Scottish MPs voting on English-only matters.
Mr Hain says Wales and Scotland would not be able to meet the challenges of the global economy if they went it alone.
He writes: "David Cameron's proposal to introduce so-called 'English votes for English laws' illustrates the danger.
"By ending the historic and fundamental principle of equality between all Members of Parliament, this plan would result in second-class status not just for MPs from Wales, Scotland and Northern Ireland, but for the citizens of those countries too."
The Union, 300 years old on Tuesday, has "shaped every aspect of our lives, in Wales just as much as in Scotland and England".
He adds: "Mounting constitutional chaos and deepening discontent would be inevitable if these plans were followed through.
"This Tory opportunism would do nothing for the people of Wales. It would only play into the hands of the separatists of Plaid Cymru and feed their fantasies, like creating a Welsh Foreign Secretary and costly embassies worldwide, while simultaneously disbanding the Armed Forces.
"That is why the Tory-nationalist double act represents such a threat. Unless we see it off it will prove a one-way street to the disintegration of the United Kingdom - and a one-way street to irrelevance for Wales."
