Britain has dismissed the latest attempt by Brussels to break the Brexit deadlock, accusing the EU side of simply trying to “rerun old arguments”.
The EU’s chief negotiator Michel Barnier insisted the UK would not be forced into a customs union against its will through the Northern Ireland backstop.
In a series of tweets following a briefing with EU ambassadors in Brussels, he said the UK would be able to exit the single customs territory unilaterally if it chose to do so.
However, in further tweets, Mr Barnier said the proposals could include "good faith/best endeavours" to ensure the UK is not locked into the backstop post-Brexit, and most importantly, if the UK unilaterally exited the customs union before other ways had been found to keep open the border on the island of Ireland, Northern Ireland would stay in the customs union.
Great Britain would leave the customs union, but Northern Ireland would remain in it, something ITV News Political Editor Robert Peston would "never" be accepted by the Democratic Unionist Party (DUP) which props up Theresa May's Government.
The apparent concession was swiftly dismissed by the DUP as neither “realistic nor sensible”.
It warned that under Mr Barnier’s proposal, Northern Ireland would still remain in the customs union – effectively imposing a “border in the Irish Sea” with the rest of the UK, something they have repeatedly said they would not allow.
ITV News said Mr Barnier's response to Mrs May's speech was "unprecedented" in "setting out the nitty, gritty details" of the negotiations on Twitter and was done "in a calculated move" to show that the EU is doing what it can to bring the deadlock to an end.
Brexit Secretary Stephen Barclay suggested the EU plan was simply a return to an earlier version of the backstop which had already been rejected by the UK.
“With a very real deadline looming, now is not the time to rerun old arguments,” he said.
“The UK has put forward clear new proposals. We now need to agree a balanced solution that can work for both sides.”
In his tweets, Mr Barnier said the EU committed “to give UK the option to exit the Single Customs Territory unilaterally, while the other elements of the backstop must be maintained to avoid a hard border”.
He said the EU was also ready to give “legal force” to reassurances given to Mrs May concerning the operation of the backstop in January in a joint letter from European Council president Donald Tusk and European Commission president Jean-Claude Juncker.
However, DUP deputy leader Nigel Dodds said Mr Barnier’s offer would annexe Northern Ireland from the rest of the UK- something Mrs May has repeatedly said that no British prime minister could agree to.
“This is neither a realistic nor sensible proposal from Michel Barnier. It disrespects the constitutional and economic integrity of the United Kingdom,” he said.
“This is an attempt to get ahead of a possible blame game and appear positive when in reality it is going backwards to something rejected a year ago.”
Watch Theresa May's speech in full
In her speech in the pro-Leave stronghold of Grimsby, the Prime Minister urged MPs to back her Withdrawal Agreement, warning the country could be plunged into crisis – and Brexit derailed altogether – if it is rejected.
“Next week, MPs in Westminster face a crucial choice: Whether to back the Brexit deal or to reject it,” she said.
“Back it and the UK will leave the European Union. Reject it and no-one knows what will happen.
“We may not leave the EU for many months, we may leave without the protections that the deal provides. We may never leave at all.”
Rejection of the deal, she added, would be a “moment of crisis”.
Her speech came after talks earlier this week between Mr Barnier and Mr Barclay and Attorney General Geoffrey Cox failed to achieve a breakthrough.
Aiming her words directly at EU leaders, Mrs May said: “Now is the moment for us to act.
"We’ve worked hard together over two years on the deal.
"It’s a comprehensive deal that provides for an orderly exit from the EU and sets the platform for an ambitious future relationship.
“It needs just one more push to address the final specific concerns of our Parliament. So let’s not hold back. Let’s do what is necessary for MPs to back the deal on Tuesday.”
Mrs May also sent a message of warning to hardline Brexiteers considering voting against her deal next week.
She said that could lead to an extension of Article 50 and possibly “a form of Brexit that does not match up to what people voted for”, or a second referendum.
Addressing workers at the Orsted wind turbine plant, she said a softer Brexit deal “could mean no end to free movement, no ability to strike our own trade deals, no end to the big annual payments, no taking back control”.
And she accused Labour leader Jeremy Corbyn of supporting “a divisive second referendum that would take the UK right back to square one”.