The Northern Irish party that supported Prime Minister Theresa May's government Friday cast its Brexit negotiation as a betrayal and warned that it could not support a deal that divided the UK.
The warning underlines May's difficulties in getting a Brexit divorce agreement, of which London and Brussels say that they have been done for 95 percent, approved by both its impetuous party and the Northern Irish lawmakers holding it to power.
Less than five months before Britain leaves the EU on 29 March, the negotiators are still negotiating a land border backup plan between the British-governed Northern Ireland and the EU member Ireland if they fail to make a deal to close.
The Democratic Unionist Party (DUP) interpreted a May promise in a letter that they would never "take effect" a UK division as a recognition that such a clause would be included in a final deal, the Times reported.
"The prime minister's letter raises alarm bells for those who appreciate the integrity of our precious trade union and for those who want good Brexit for the entire UK," said DUP leader Arlene Foster.
"From her letter it seems that the prime minister is stuck to the idea of a border along the Irish Sea with Northern Ireland in the regulatory EU regulatory system," Foster said.
May, which announces the centenary of the end of the First World War during ceremonies in Belgium and France on Friday, has repeatedly said that it will not accept any deal that divides the United Kingdom. But the DUP has torpedoed Brexit conversations before.
Almost a year ago, the refusal to close a deal at the border caused the temporary collapse of the Brexit talks in a crucial phase. Negotiators later found a way to keep all sides on board.
The Brexit deal – or lack of it – will shape the prosperity of Britain for many generations and have long-term consequences for the influence of the European Union.
Both parties need an agreement to ensure that trade continues to flow between the world's largest trading bloc and the fifth largest global economy. The other 27 members of the EU together have about five times the economic power of Great Britain.
"A successful outcome is not guaranteed, but I think it's possible in the next few weeks," said Irish Prime Minister Leo Varadkar.
But May has struggled to disentangle nearly 46 years of membership without harming the trade or shocking the lawmakers who will ultimately determine the fate of any deal they can close.
If a deal is voted down by the parliament, the United Kingdom would be put into an uncertain future: without a deal, the collapse of the May government, an election or, some opponents of the Brexit hope, a new referendum.
Since formal negotiations began in May 2017, negotiators have had difficulty finding a solution for the 310-mile land border on the island of Ireland.
Both sides want to avoid customs posts that some fear to reenact violence in Northern Ireland that was ended by a peace agreement concluded in 1998 in the United States.
Under discussion there is a fall-back option for the border. London argues that the UK should remain in a temporary customs regime with the EU, although it is unclear how long and what will happen after it leaves.
The EU has insisted on an insurance policy – or a backstop for the backstop – a requirement that London sign a treaty clause that could leave Northern Ireland within the EU Customs Area.
That is unacceptable to May's "Unionist" supporters, whose entire political ideology is based on the defense of Northern Ireland in the United Kingdom.
DUP leader Foster said in a letter to May that a backstop can not bring Northern Ireland to specific sectoral EU market rules. And every backstop should have a clear expiration date, she said.
Without DUP support for a deal, May has much less chance to get her deal through parliament. She needs 320 votes to get a deal.
"The PM knows what the consequences are, they should now reconsider," said DUP Lawmaker Sammy Wilson.
Since her failed bet on a quick election in 2017, her party lost her majority in parliament, May relied on the DUP to rule. May has 315 legislators, but relies on the 10 DUP legislators to implement legislation.
The American bank JP Morgan said she expected the parliament to reject a deal on the first attempt, but would later agree to a second or third attempt.