An EU proposal for the Northern Ireland border threatens the “constitutional integrity” of the United Kingdom, Theresa May has said, the BBC reports.
A draft legal agreement published by the EU proposes a “common regulatory area” on the island of Ireland after Brexit, if solutions cannot be found.
Mrs May said “no UK prime minister could ever agree” to this.
The EU says the controversial “backstop” option is not intended to “provoke” the UK.
Unveiling the draft agreement, EU chief negotiator Michel Barnier called on the UK to come up with alternatives.
He said the text was “no surprise” and was just a legally-worded assessment of what had been agreed so far.
Mr Barnier said the document contained “concrete and realistic solutions” in relation to the question of how to avoid a hard border once the UK leaves the EU’s customs union.
Other options – a UK-EU deal that means checks are not needed and technological solutions – will also be explored.
Speaking at Prime Minister’s Questions, Mrs May said she would be making her opposition to the “backstop” option “crystal clear” to the EU commission.
“The draft legal text the commission have published would, if implemented, undermine the UK common market and threaten the constitutional integrity of the UK by creating a customs and regulatory border down the Irish Sea, and no UK prime minster could ever agree to it,” she said.