By Guy Faulconbridge and Sarah Young
LONDON (Reuters) - The Northern Irish party which props up Prime Minister Theresa May's government on Friday cast her Brexit negotiation as a betrayal and cautioned it could not support a deal that divided the United Kingdom.
The warning underscores the travails that May faces in getting any Brexit divorce deal, which London and Brussels say is 95 percent done, approved by both her fractious party and by the Northern Irish lawmakers who keep her in power.
Less than five months before Britain is due to leave the EU on March 29, negotiators are still haggling over a backup plan for the land border between British-ruled Northern Ireland and EU member Ireland should they fail to clinch a deal.
The Democratic Unionist Party (DUP) interpreted a promise made by May in a letter that she would never let a division of the UK "to come into force" as an admission 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 value the integrity of our precious union and for those who want a proper Brexit for the whole of the United Kingdom," DUP leader Arlene Foster said.
"From her letter, it appears the prime minister is wedded to the idea of a border down the Irish Sea with Northern Ireland in the EU single market regulatory regime," Foster said.
The DUP has torpedoed Brexit talks before.
Nearly a year ago, its refusal to sign off on a deal on the border caused the temporary collapse of Brexit talks at a crucial stage. Negotiators later found a way to keep all sides on board.
The Brexit deal - or the lack of one - will shape Britain's prosperity for generations to come and have long-term consequences for the European Union's global clout.
Both sides need an agreement to keep trade flowing between the world's biggest trading bloc and the fifth largest global economy. The other 27 members of the EU combined have about five times the economic might of Britain.
Britain's junior Brexit minister told a German media outlet he was optimistic there would be a deal soon.
"ALARM BELLS"
Ever since May triggered formal divorce talks in March 2017, negotiators have struggled to find a solution to the 310-mile land border on the island of Ireland.
Both sides want to avoid customs posts that some fear could reignite the violence in Northern Ireland that was ended by a U.S.-brokered peace accord in 1998.
Under discussion is a fallback option for the border. London argues that the whole of the United Kingdoms should stay inside a temporary customs arrangement with the EU, though it is unclear for how long and what would happen after exiting that.
The EU has insisted on a insurance policy - or a backstop to the backstop - a demand London signs a treaty clause that could leave Northern Ireland inside the EU customs space.
That is unacceptable for May's "Unionist" supporters whose entire political ideology is based on defense of Northern Ireland's place within the United Kingdom.
"We will not support arrangements that leave Northern Ireland separated from the rest of the United Kingdom and tied to the European Union's customs or regulatory regimes," Foster said in a letter to May which was published on Friday.
She added that any backstop could not leave Northern Ireland aligned to specific sectoral EU market regulations. And any backstop would have to have a clear expiry date.
If May can clinch a deal with the EU, she still needs to get that deal approved by parliament. But it is unclear if she has the numbers: Around 320 votes in the House of Commons are needed to be certain of winning a vote.
Ever since her botched gamble on a snap election in 2017 lost her party its majority in parliament, May has relied on the DUP to govern.
May's Conservative Party has 315 lawmakers. It relies on the 10 DUP lawmakers to pass legislation.
"The PM knows the consequences, she now needs to reconsider," DUP lawmaker Sammy Wilson said.