The Democratic Unionist Party leader Arlene Foster has said she will not support British Prime Minister Theresa May's decision to have a Northern Ireland specific Brexit backstop which will see a border down the Irish Sea.
The DUP, upon whose support Mrs May relies, will not support that and will not be able to support it in the British parliament, Ms Foster said.
The party accused the prime minister of breaking a promise that she would never sign up to a deal that treated Northern Ireland differently from the rest of the UK.
The party has been angered by a leaked letter it was sent in which Mrs May said EU negotiators were still pushing for Northern Ireland to remain in the single market and customs union if talks collapse.
"Our problem with the correspondence is that she has essentially ... confirmed that there's going to be a Northern Ireland specific backstop and indeed that as regards her negotiating position, she's going to have Chequers for the whole of the United Kingdom. In other words, we will have a different regulatory regime from the rest of the United Kingdom and essentially there is going to be a border down the Irish sea and no unionist would be able to support that," Ms Foster told RTÉ News.
"It is very clear to me that Theresa May will have a job of work to do if she is going to get this through Cabinet," said Ms Foster.
When asked if the backstop was an insurance policy that may never be used, Ms Foster said: “Well if that’s the case, why have it there in the first place. Why risk the break up of the United Kingdom which she says she cares about very deeply”.
“As Unionists, we have to be clear with the Prime Minister that we’re not going to support it,” she added.
Ms Foster said too she did not agree with Taoiseach Leo Varadkar that a backstop would be required.
“They (Dublin) feel they need the backstop to ensure there is no hard border. I would dispute that because who is going to put up the hard border. Certainly not the United Kingdom government, we don’t want a hard border, the Irish Government does not want a hard border so who exactly is going to put up a hard border ... what we need is a trading relationship that works for everybody,” she said.
When asked about the leaked letter to the Times newspaper, Ms Foster said it hadn't been leaked by the DUP. "I don't know why it has been leaked - I can't think of the raison d'etre as to why a letter like this would be leaked, particularly when we're at such a key time," she said.
Ms Foster said Mrs May will now have to consider whether or not she can rely on the support of the DUP in the weeks ahead.
“She now has to decide whether she wants to proceed down this road where she won’t have the support of the ten DUP MPs in Westminster. She has to do the maths as to whether she can proceed without our support in parliament. That’s for her to decide as to what she will do next,” she said.
Separately, Ms Foster told BBC news there were “many others” in the Conservative Party who could also not support the prime minister’s proposals, as set out in the leaked letter.
“Not only would we not be able to support what she has said to us but there are many others who would not be able to support it in her own party as well.”
She also denied her party’s confidence and supply deal with the Tories was on “shaky ground”.
“I don’t think it leaves it on shaky ground because of course the confidence and supply agreement was entered into at a time of great national instability, we wanted to see stability in the government at that time and we also wanted to deliver on a Brexit vote that had been taken.
“We don’t believe that the prime minister’s letter shows that we are delivering on that Brexit vote, so we will have to revisit all of that if this goes to a meaningful vote.”
She added: “We are writing back to the prime minister in relation to her correspondence and that will be done tonight.”
Earlier DUP MP Sammy Wilson told RTÉ’s Sean O’Rourke Programme “the battle is not lost. Even if she gets it [her Brexit plan] through the Cabinet, she will lose it in the House of Commons.
“There will be consequences if the [BRITISH]government breaks its side of the agreement.”
“This letter raises concerns. But nothing is proposed yet, we will make a judgment on the facts of the case.”
He said the British government had to abide by the vote of the people, who had voted for Brexit. He went on to warn that a no deal scenario would be “extremely damaging” to the economy of the Republic of Ireland.
Mr Wilson said he was concerned that once a legal backdrop is signed up to, “we know the direction that the EU will be heading in. They will veto it ever being removed.”
“That’s the harsh reality. All the prime minister’s points (in the letter) are aspirational. The back stop will be legal.”
The warning from the DUP underscores the travails Mrs May faces in getting any Brexit divorce deal, which London and Brussels say is 95 per cent done, approved by both her fractious party and by the Northern Irish lawmakers who keep her in power.
A Downing Street spokesman said: “The prime minister’s letter sets out her commitment, which she has been absolutely clear about on any number of occasions, to never accepting any circumstances in which the UK is divided into two customs territories. The government will not agree anything that brings about a hard border on the island of Ireland.”
The EU has insisted on a Northern Ireland-only “backstop to the backstop” in case negotiations on a wider UK approach break down. It is designed to avoid the return of a hard border between the Republic and Northern Ireland.
Any version of the backstop would apply unless and until a wider UK-EU deal on the future relationship solved the issue of how to avoid a hard border.
Ever since Mrs May triggered formal divorce talks in March 2017, negotiators have struggled to find a solution to the 400km land border on the island of Ireland.
The current proposal would see the whole of the United Kingdom remain inside a temporary customs arrangement with the EU, though it is unclear for how long and what would happen after it exits that.