DUP accuses May of breaking Brexit pledge over 'Irish Sea border'

PM says in letter she would not let break-up of UK customs territory ‘come into force’.

Theresa May’s Brexit plans have come under fresh doubt after her Democratic Unionist party allies (DUP) said a letter from her “raises alarm bells”, suggesting she will not be able to depend on its support to get the proposals through parliament.

The party accused the prime minister of breaking a promise that that she would never sign up to a deal that treated Northern Ireland differently from the rest of the UK. It has been angered by a letter in which May said EU negotiators were still pushing for Northern Ireland to remain in the single market and customs union if talks collapse.

The DUP has seized on a particular paragraph– which has been seen by the Times – in which May said she could not accept circumstances or conditions that could break up the UK customs territory to come into force.

The DUP leader, Arlene Foster, tweeted on Friday: “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. From her letter, it appears the PM is wedded to the idea of a border down the Irish Sea with NI in the EU SM regulatory regime.”

The leak of the letter is seen by some observers, as well as the DUP, as part of a laying of the ground by May for a showdown with the party over checks in British ports or factories in Northern Ireland or Britain.

Under the EU proposals, UK officials would be “competent authorities” to conduct the checks, but to EU rules, something the DUP has said will cross its red line.

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The DUP MP Sammy Wilson heaped further pressure on the government, telling the BBC Radio 4’s Today programme that the letter was sent to the party but was put into the public domain by Downing Street. “I assume it’s part of the precess of trying to get into the public domain what will finally be agreed,” he said.

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.”

The EU has insisted on a Northern Ireland-only “backstop to the backstop” in case negotiations on a wider UK approach break down. Any version of the backstop would apply unless and until a wider UK-EU deal on the future relationship solved the issue of how to avoid a hard border with Ireland.

The row comes as Brexit is expected to dominate the agenda of the British Irish Council, which will be attended by the taoiseach, Leo Varadkar, David Lidington, who is in effect May’s deputy, and the Northern Ireland secretary, Karen Bradley. The summit on the Isle of Man will also be attended by the first ministers of Scotland and Wales, Nicola Sturgeon and Carwyn Jones.

Brexit is also expected to be a focal point when May meets the French president for a working lunch after attending armistice commemorations in Belgium and France.

Downing Street has played down suggestions that a Brexit deal is imminent, after the European council president, Donald Tusk, appeared to indicate a breakthrough could come within the next week.

Meanwhile, Eurosceptic Conservative MPs have said they will still vote down the government’s Brexit deal, even if May negotiates an exit clause from the Irish backstop, according to the former minister Steve Baker.

The cabinet has been locked in a bitter internal wrangle about whether, and how, the government could extricate itself from the backstop, with some ministers concerned May’s plans could leave the UK in a permanent limbo.