- Video report by ITV News Political Correspondent Romilly Weeks
Theresa May has said "no UK Prime Minister could ever agree" to sign up to the Brexit legal plans drawn up by the EU which could see Northern Ireland diverge from Great Britain to remain aligned with Europe.
The Prime Minister said the the text would "threaten the constitutional integrity of the UK" and she would be making it "crystal clear" to Commission president Jean-Claude Juncker that she would not accept it.
It comes after the EU published its draft Article 50 Withdrawal Agreement including the highly controversial for Northern Ireland proposals as a "backstop" measure to ensure there is no hard border with the Republic.
Speaking at Prime Minister's Questions in the House of Commons, Theresa May said the Government was "absolutely committed to ensuring that we deliver on no hard border between Northern Ireland and Ireland".
But she said the UK could never implement proposals to leave Northern Ireland aligned with Brussels instead of London.
The Irish question has become a major stumbling block to progress as the UK prepares to enter a next round of talks to a transition deal.
Brussels' chief negotiator Michel Barnier today has today laid out the EU plans, which include proposals to establish a "common regulatory area" between the EU and Northern Ireland after Brexit.
He said that the plans should be no surprise to anyone, and were only intended to be used a "backstop" measure if no other solution to avoid a hard border with the Republic could be found.
- Video report by ITV News Europe Editor James Mates
But the proposals were attacked as an assault on the UK's integrity, with former Brexit minister David Jones saying they amounted to an attempt to annex Northern Ireland.
Fears were also stoked after prominent Brexiteer Boris Johnson sent a letter to Mrs May suggesting there may be a physical border in Ireland and the focus should be on ensuring it does not become "significantly" harder.
Today, he claimed that Remainers were using the issue to try to "frustrate" Brexit.
Former Conservative Prime Minister Sir John Major said that Mrs May should offer MPs a free vote on the final Brexit deal, with the option of putting it to the public in a second referendum.
He urged her to stand up to the "ultra-Brexit" minority in her party and drop her "red lines" of taking Britain out of the single market and customs union.
"Unrealistic aspirations are usually followed by retreat. That is a lesson for the negotiations to come," he warned.
"They will be the most difficult any Government has faced. Our aims have to be realistic. I am not sure they yet are."
Labour leader Jeremy Corbyn today taunted Mrs May over what he said were contradictory and confused messages over Brexit from her government.
"This is a government in disarray," he said. "
"Every time the cabinet meets, all we get is even more bizarre soundbites.
"Remember when we had ‘Brexit means Brexit’? Then we had ‘red, white and blue Brexit’, which presumably appealed to the members opposite, then we had ‘liberal Brexit’, and now we have ‘ambitious managed divergence’.
"The government is so divided the Prime Minister is incapable of delivering a coherent and decisive plan for Brexit."