The EU has proposed a “common regulatory area” on the island of Ireland if solutions cannot be found for the post-Brexit border.
Unveiling a draft legal agreement, EU negotiator Michel Barnier said this would be a “backstop” option and 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 has been agreed so far.
But there has been criticism in the UK at the proposal for Northern Ireland.
The DUP’s Westminster leader Nigel Dodds said his reaction to the publication was one of “amazement” that the EU thought it “could possibly fly with either us or the British government”.
“We did not leave the European Union to oversee the breakup of the United Kingdom,” he told the BBC, adding that it would be “catastrophic” for Northern Ireland to be “cut off” from UK markets.
Conservative Brexiteers have also said it is “completely unacceptable” and would effectively annex Northern Ireland.
But Mr Barnier said the document contained “concrete and realistic solutions” in relation to 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.
Mr Barnier denied that the inclusion of the “backstop” option was designed to provoke the UK.