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. London`s latest political spat underscores the trouble May`s fractured government faces in passing through parliament any Brexit arrangement it thrashes out with Brussels over the coming days. The Times newspaper reported that May sent a five-page letter on Tuesday to the leaders of Northern Ireland`s small Democratic Unionist Party (DUP) that props up her government. May reportedly told her ruling coalition partners she would never allow the disputed Brexit deal proposal offered by Brussels to "come into force". But DUP leaders said on Friday that May`s wording meant the 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 it never would -- and they threatened to vote against the agreement. "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. DUP Brexit spokesman Sammy Wilson told Sky News that May was guilty of "total betrayal". "If a deal emerges shaped on the contents of the PM’s letter, @duponline MPs & our allies will not support it," Wilson tweeted. "The PM knows the consequences, she now needs to reconsider." `Showdown` At issue is the 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-back 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. It would then require additional checks on goods and agriculture flowing between Northern Ireland and mainland Britain along the Irish Sea. The DUP views this as a red line and a betrayal of the peace agreement that brought peace to the troubled province in the 1990s. Some British media said May`s government leaked the letter to prepare the ground for what The Guardian called a final "showdown" with the DUP over the checks. Culture Secretary Jeremy Wright told BBC Radio that May`s government would not let the EU carve up the United Kingdom. "There is no doubt we have understood the concern the DUP and others have expressed about a hard border down the Irish Sea, we share it, and we will not accept a deal that involves that component." Eurosceptics in May`s 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 failure to secure DUP`s backing could see the government lose the Brexit vote and potentially face early elections.