Theresa May faced a fresh Brexit headache as her plans risked a rift with her Democratic Unionist Party allies.
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 European Union’s fallback proposal aimed at avoiding a hard border between Ireland and the UK would effectively keep Northern Ireland aligned with Brussels’s customs union and single market.
A report in The Times on Friday claimed that a leaked letter from May to Foster and her deputy Nigel Dodds set out the PM’s approach.
She 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.
Meanwhile, a long-awaited Brexit deal could be brokered within a week, senior EU leader Donald Tusk has claimed.
Sparking fervent denials from Downing Street, the European Council President told Channel 4 News on Thursday there could be a breakthrough in negotiations in as little as “maybe five, maybe six, maybe seven days”.
In the letter to the DUP, 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 despite Mrs May’s insistence it will never come into effect.
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 as she attempts to get a 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.”
Tusk’s comments came on the same day Austrian newspapers reported that Brexit Secretary Dominic Raab and EU chief negotiator Michel Barnier could meet over the next few days to seal an agreement.
But Downing Street attempted to pour cold water on the claims of a weekend deal, stressing that no agreement had yet been reached.
“We are still in negotiations, and on that basis we don’t know when and if this will conclude,” a spokesman said, adding that a much-rumoured Cabinet meeting to allow ministers to sign off the deal had not been scheduled.
Meanwhile, a senior government source said that any reports in the European media that a deal could come in the next few days should be taken “with a very large pinch of salt”.
It comes after the PM last month told MPs that 95% of the deal had been agreed, with the key sticking point of the “backstop” to prevent a hard border between Ireland and Northern Ireland remaining unresolved.
May’s plan would see the whole UK effectively agree to remain in the customs union to help avoid a hard border with Ireland as a backstop if no other arrangement can be found.
However, fears that this could become a permanent settlement led the Labour Party and Tory Brexiteers - including Environment Secretary Michael Gove - to demand that May publishes the full legal advice setting out how the arrangement could be ended.
While Dominic Grieve wrote to Cabinet Secretary Sir Mark Sedwill calling for government documents explaining the final withdrawal agreement to be made public, former Brexit Secretary said how the UK could exit from the customs union must be “pinned down” before MPs and peers vote on the deal.