The DUP leader Arlene Foster has said that she will not support British Prime Minister Theresa May’s decision to have a Northern Ireland specific backstop which would see a border down the Irish Sea.

In an interview with RTÉ News, Ms Foster said the problem with Mrs May's letter is that she has confirmed there will be a Northern Ireland specific backstop and the Chequers plan for the rest of the UK.

Ms Foster said there will be a different regulatory regime for Northern Ireland from the rest of the UK and there will be a border down the Irish Sea.

She said the DUP will not support that and will not be able to support it in the British parliament.

Ms Foster said it was clear to her that Mrs May will have "a job of work to do to get this through cabinet".

She said it causes her "great angst" to think that Northern Ireland would be a rule taker from the EU but would have no say around those rules. 

Ms Foster added that the proposal would cause a "democratic deficit" and could cause Northern Ireland to diverge away from the United Kingdom over the next decade. 

She said her party had "only one red line" and that the whole political rationale of the DUP was to protect the union. 

Ms Foster said the Irish Government does not need a backstop to prevent a hard border, what they need is a trading relationship.

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She said that Mrs May now needs to decide if she wants to pursue this without the support of the ten DUP MPs.

She also said the leak of the private correspondence between the DUP and Mrs May came from London and she did not know what the motivation was for the leak.

Sinn Féin has called on the DUP to put the interests of the people of the North above their party political interests.

MLA Máirtín Ó Muilleoir said the DUP has ignored the fact that the democratic majority of people in the North voted to stay in the EU and that the majority of MLAs support the backstop.

He said the backstop was not the best solution but was the only solution on offer and the DUP needed to agree to it.

Earlier, the DUP said alarm bells began to ring when a letter was received from Mrs May this week.

The leaked document seen by The Times of London was in response to a letter sent by the DUP to Mrs May on 1 November and suggests the EU is pushing for a customs border in the Irish Sea between Northern Ireland and the rest of the UK if Brexit trade talks fail.

This is to ensure no return to a hard border on the island of Ireland.

Mrs May told Ms Foster and deputy DUP leader Nigel Dodds that she would not allow a divide between Northern Ireland and the rest of the UK to "come into force".

That has led the DUP to believe that the clause could be in the Brexit agreement, which could be announced within days.

Mrs 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 DUP said it is "totally unacceptable that there could be a Withdrawal Agreement which provided that Northern Ireland at any time in the future could be subject to the rules of the Customs Union or parts of the Single Market whilst the rest of the UK was not".

The letter, from Mrs Foster and Mr Dodds, said an agreement like that would mean that Northern Ireland "must follow rules and laws determined by other countries with no say in their formulation and with a goods border in the Irish Sea". 

The letter adds that the constructive engagement strategy of the UK government's negotiators has not always been reciprocated by the EU. On the backstop, it says the EU has ignored what was agreed and "instead proposed what it wished was agreed".

The DUP, it says, appreciates that the Withdrawal Agreement is legally binding. 

"Unlike the previous political declarations, the scope for delay, fudge or obscurantist language has passed. This is now a time for clarity and plain speaking," the letter states. 

"We are resolutely opposed to any Northern Ireland specific backstop which not only leaves Northern Ireland aligned to specific sectoral EU market regulations but is, when operational, time-limited by date."

The DUP letter notes that if Northern Ireland is subjected to EU single market regulations then it will also be subject to the ECJ.

It says that this would leave Northern Ireland as a "rule taker from Brussels" and would create "a democratic deficit" for the manufacturing and agriculture sectors of their economy. 

It says the risk of a hard border has been inflated and that Scottish nationalists are "ready to pounce" on distinctive treatment for Northern Ireland. 

The EU's fallback proposal aimed at avoiding a hard border would effectively keep Northern Ireland aligned with Brussels' customs union and single market.

In the letter seen by The Times, Mrs 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."

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.

Mrs 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."

Such an inclusion would create severe difficulties for the Mrs May with the DUP, as well as with Brexiteers in her own party.

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The DUP's Brexit spokesperson has accused Downing Street of leaking the letter "as part of a process to get into the public domain what will finally be agreed" in Brexit.

Sammy Wilson called for the British government to make available the legal advice it has been given in negotiating the Withdrawal Agreement "for the sake of clarity".

Speaking on BBC Radio 4, Mr Wilson said the DUP wants "to trust" Mrs May, but added "you have to judge any promise by what is actually delivered by an agreement".

He said the letter made it clear that a Northern Ireland-only backstop that will require specific alignment for regulations is being considered.

He said the promise made in paragraph 50 of the Joint Report in December that allows the Northern Ireland Assembly to have a say in what is agreed seems to have been removed, describing it as a breach of a promise given that would ensure that "Northern Ireland would not be cut off from rest of the UK".

A Downing Street spokesman said Mrs May 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 to anything that brings about a hard border on the island of Ireland."

He said there are already arrangements in place such as the single electricity market and on animal health "that reflect the unique relationship between Ireland and Northern Ireland, and we want to make sure nothing damages the integrity of the UK's own internal market, and that businesses in Northern Ireland can continue to access it as they do now, and that's what our proposals deliver".

Additional reporting: Fiona Mitchell/ PA