British Prime Minister Theresa May drew the fury of her crucial Northern Irish allies on Friday after seemingly accepting an EU-backed Brexit solution they fervently oppose.
In it, May reportedly tried to assure her allies that she would never allow a Brexit deal proposal offered by Brussels to "come into force".
But DUP leaders said on Friday that May's wording meant the Brexit fix would still be included in the withdrawal agreement that London and Brussels hope to reach in the coming days.
They said May had earlier promised them that it never would.
"The PM's letter raises alarm bells for those who value the integrity of our precious union & for those who want a proper Brexit for the whole UK," DUP leader Arlene Foster tweeted.
"From her letter, it appears the PM is wedded to the idea of a border down the Irish Sea with NI in the EU SM regulatory regime." DUP Brexit spokesman Sammy Wilson told Sky News that May was guilty of "total betrayal".
At issue is the vexing problem of how to avoid border checks between British Northern Ireland and the Republic of Ireland after Brexit enters into force on March 29.
London suggests Britain could temporarily stay aligned with the bloc's trade rules but wants to reserve the right to exit the arrangement.
The EU appears ready to accept that -- but only if there is a fall-backed option written into the withdrawal agreement.
This so-called "backstop to the backstop" would see Northern Ireland become wedded to the EU single market and customs union should London and Brussels fail to strike a permanent trade deal.
This would then require additional checks on goods and agriculture flowing between Northern Ireland and mainland Britain along the Irish Sea.
The disagreement underscores the difficulties a smooth Brexit faces even if London and Brussels agree the terms of a divorce deal.
May is due to meet French President Emmanuel Macron on Friday on the sidelines of World Way I commemorations being held in Belgium and France.
But she needs the DUP support at home once the deal -- if any -- comes up for approval in parliament.
Some eurosceptics in her Conservative Party are already threatening to vote against the deal because it could lock Britain into a long-term customs arrangement with the EU.
May's loss of DUP backing could see the government lose the Brexit vote in parliament and potentially face early elections.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)