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...