The DUP has accused the PM of breaking promises over plans to avoid a hard border between Northern Ireland and the Republic of Ireland post-Brexit.
The party reacted angrily to a letter from Theresa May to DUP leader Arlene Foster, leaked to the Times.
Mrs Foster claimed the PM was “wedded to a border down the Irish Sea” as a fallback option for avoiding checks, if no free trade deal is reached in time.
Downing Street reiterated the PM’s own commitment to avoiding a hard border.
Mrs May relies on DUP support in key votes because she does not have a majority in the House of Commons.
Negotiating a backstop – an insurance policy to keep an open border between Northern Ireland and the Republic of Ireland until the UK and EU agree their future relationship – remains the main obstacle in the way of a wider agreement between the two sides.
Mrs May describes it in her letter to the DUP as “an insurance policy that no-one in the UK or the EU wants or expects to use”.
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If sufficient progress is made on the issue in the next few days, it is thought a special cabinet meeting could be held early next week for ministers to approve the draft agreement on the terms of the UK’s exit.
A Downing Street spokesman said the letter sets out the prime minister’s commitment “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 spokesman added.
Culture Secretary Jeremy Wright told the BBC that “these things take time to get right” and Mrs May should be given the space to do a deal which protects the UK and Northern Ireland’s interests.
What has annoyed the DUP?
The BBC’s John Campbell said what was upsetting the DUP is the issue of single market regulations, not customs.
He said the letter suggested the unique circumstances of Northern Ireland may require alignment with EU single market regulations “in some scenarios” adding that any such alignment would have to be “carefully circumscribed to what is strictly necessary to avoid any hard border”.
Mrs Foster told the Times: “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 five-page letter was sent three days ago. It states that the EU has proposed a contingency plan to keep Northern Ireland tied to the EU customs union and single market, if a future free trade deal is not ready in time.
In it, Mrs May writes that she could not accept any circumstances in which the contingency plan “could come in to force”.
But the DUP has interpreted the wording of her letter to mean that the EU’s proposal – known as the “backstop to the backstop” – will be contained in the Brexit divorce deal, despite Mrs May’s insistence it will never come into effect.
The DUP MP Sammy Wilson told BBC News “She has broken our promise to us, she’s broken her promise to the people of the UK.”
BBC political correspondent Iain Watson said the row came down to an issue of trust between Mrs May and her DUP allies, who were suspicious she might sign up to a deal with the EU they did not agree with.
What is the backstop?
The UK and the EU both want to avoid a “hard border” – physical checks or infrastructure between Northern Ireland and Ireland.
The backstop solution is a safety net – an arrangement to maintain an open border on the island of Ireland if a 21-month post-Brexit “transition period” ends before a final trade deal is ready to come into force.
The UK government has proposed a backstop which would effectively keep the whole of the UK in the EU customs union for a limited period after Brexit. Brexiteers have pushed for it to include an expected end date of 2021.
And the backstop to the backstop?
The EU is adamant that there cannot be a time limit and any backstop must apply “unless and until” it is no longer needed.
So if a proposal for a time-limited, all-UK backstop formed part of an agreement there would also need to be something in place in case the issue of the Irish border was not sorted out by that end date.
That “something” is a return to the original EU proposal which would see Northern Ireland in a European customs union and would mean a customs border in the Irish Sea.
Brexit: DUP accuses May of breaking promises on Irish border}