Theresa May faces a fresh Brexit headache after a letter to the DUP on the backstop risked a rift with her unionist allies.
Following receipt of the letter, DUP leader Arlene Foster said the prime minister appeared "wedded to the idea of a border down the Irish Sea" despite Downing Street's assurances to the contrary.
The letter suggests the EU's fallback proposal aimed at avoiding a hard Border between the North and the Republic, regardless of whether the UK and the EU reach a Brexit deal, would effectively keep Northern Ireland aligned with Brussels's customs union and single market.
The leaked letter from Ms May to Ms Foster and her deputy Nigel Dodds sets out Ms May's approach.
It states that Ms May wants a “backstop” measure which would create a temporary “joint customs territory” with the EU for the whole of the UK.
But Brussels appears set to insist on a Northern Ireland-only "backstop to the backstop" in case negotiations on a wider UK approach break down.
In the letter, obtained by The London Times, Ms May said: “I am clear that I could not accept there being any circumstances or conditions in which that ‘backstop to the backstop’, which would break up the UK customs territory, could come in to force.”
But the DUP has interpreted the wording of her letter to mean that the measure will be contained in the Brexit divorce deal.
Ms Foster said: “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 UK.
"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."
The prime minister relies on the support of the DUP’s 10 MPs for her Commons majority, votes which may become crucial when she attempts to get a Brexit deal through parliament.
A Downing Street spokesman said: “The prime minister’s letter sets out her commitment, which she has been absolutely clear about on any number of occasions, to never accepting any circumstances in which the UK is divided into two customs territories.
“The government will not agree anything that brings about a hard Border on the island of Ireland.”
Any version of the backstop would apply unless and until a wider UK-EU deal on a future relationship resolved the issue of how to avoid a hard Border in Ireland. – PA