Theresa May was accused of breaking her promise to the DUP that she would never sign a Brexit deal separating Northern Ireland from the rest of the UK.
A letter from the Prime Minister was interpreted by DUP chief Arlene Foster as saying that Northern Ireland would be separated from the rest of the United Kingdom by a customs border in the Irish Sea in the event of a no-deal brexit.
The Prime Minister's letter raises alarm bells for those who value the integrity of our valuable union and for those who want a proper Brexit for the entire United Kingdom. Her letter states that the Prime Minister is bound by the idea of a border between the Irish Sea and NI in the framework of the EU regulatory regime. https://t.co/qN6c41zx7U
– Arlene Foster (@DUPleader) November 9, 2018
2/2 November 1, 2018 Letter from Arlene Foster @ DUPleader & Nigel @NigelDoddsDUP to the prime minister. "It is crucial for both NI and the whole of the United Kingdom that there is clarity in an agreement and that there can be no divergence within the United Kingdom for future generations." pic.twitter.com/gCcvp01p8O
– DUP (@duponline) November 9, 2018
According to The Times, which received the five-page letter also addressed to DPC Vice-Duty Chair Nigel Dodds, Ms. May said the EU was still pushing for a "backstop lock-back", but insisted that it did not allow a split into effect "between Northern Ireland and the rest of the United Kingdom.
The newspaper said that this was interpreted by the DUP as meaning that the EU proposal would become part of the legally binding resignation agreement.
"The letter from the Prime Minister warns the people who value the integrity of our precious Union and those who want a proper Brexit for the entire UK," Ms Foster told the Times. "It appears that the Prime Minister is linked to the idea of a border along the Irish Sea with Northern Ireland in the framework of the EU regulatory system for the internal market."
Ms. May had previously stated that she would never sign a Brexit deal that would create barriers between Northern Ireland and the UK.
It was one of the "red lines" that the DUP had warned the Prime Minister not to cross if she wanted her MPs to continue to support the Conservatives in Westminster under their confidence and supply agreement.
Without the support of the 10 DUP deputies on important votes such as budgets, Ms. May and her government face great difficulties, not least in getting their Brexit plan through Parliament.
The leaked letter also suggests that the whole of the UK could remain closely linked to the EU, as the government "does not expect the regulations between the UK and Northern Ireland to differ".
In her letter, Ms. May wrote that the retention rate was "an insurance policy that nobody in the UK or the EU wants or wants to use," and that this would not last indefinitely.
A Downing Street spokesman told The Times that the PM would not accept anything that sets Northern Ireland apart from the rest of Britain.
"The letter from the Prime Minister sets out her pledge, which made her absolutely clear on every occasion that she would never accept circumstances in which the United Kingdom is divided into two customs territories, and the government will not agree on anything that leads to a harsh result Border on the island of Ireland. "