The party’s leader raged “no unionist” could support the Prime Minister’s apparent advocacy of a Withdrawal Agreement that includes a Northern Ireland-specific backstop measure to avoid a hard border on the island of Ireland. Theresa May relies on the support of the 10 MPs from the DUP for her to gain a crucial majority in the Commons, votes which could ultimately determine whether Mrs May gets any Brexit deal through Parliament. Mrs Foster accused Mrs May of being “wedded to the idea of a border down the Irish Sea”, despite assurances from Downing Street stating otherwise.
In a letter to the DUP, the Prime Minister insisted such a backstop would never materialise, and says she wants a “backstop” measure which would create a temporary “joint customs territory” within the European Union for the whole of the UK.
But Mrs Foster warned the DUP is massively against the divorce deal from Mrs May that could see Northern Ireland operate under a different regulatory agreement to the rest of the UK.
Stormont’s first minister also claimed there are “many others” in the Prime Minister’s own Conservative Party who may also not support her plans.
Mrs Foster said: “We would not be able to support this if it came to Parliament in the form that it is in the letter.
“There are stages to go through before it comes to Parliament. She still has to have a cabinet meeting in relation to this matter and we believe there is a chance for her to reflect on the fact we will not be able to support it in its current form.”
In an interview with the BBC, Mrs Foster was asked if she trusts the Prime Minister.
She replied: “It’s not a question of trusting the Prime Minister, it is a question of what her proposals are for exiting the European Union.
“She has sent us where she believes she is currently at, and remember this is before she goes to Brussels to negotiate with them on what they believe is possible, but currently, as it stands, we could not support her proposals.”
Mrs Foster added: “Not only would we not be able to support what she has said to us but there are many others who would not be able to support it in her own party as well.”
The DUP’s chief whip Sir Jeffery Donaldson also warned Mrs May she cannot rely on support from the party’s MPs when she puts her deal to a vote in the Commons.
Sir Jeffrey told BBC Radio 4: “If we think a Brexit deal is not good for the UK, we will say so. Let’s see what the deal is. We’re not afraid of a general election.
“I’m quite happy to go to the people of Northern Ireland on the basis that we voted against a deal because it was not in the interests of Northern Ireland. We want the UK to stay together, and we are not alone in this.”
Despite the scathing assessment of Mrs May’s plans for an Irish backstop, the DUP leader denied suggestions her party’s confidence and supply deal with the Conservatives was on “shaky ground”.
She said: “I don’t think it leaves it on shaky ground because of course the confidence and supply agreement was entered into at a time of great national instability, we wanted to see stability in the Government at that time and we also wanted to deliver on a Brexit vote that had been taken.
“We don’t believe that the Prime Minister’s letter shows that we are delivering on that Brexit vote, so we will have to revisit all of that if this goes to a meaningful vote.”
In the leaked letter to the DUP, obtained 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.”
But she added the “unique circumstances” of Northern Ireland “could require specific alignment solutions in some scenarios” on regulations, while the scope of any alignment with EU rules would be limited to what is “strictly necessary” to avoid a hard border.
The DUP has claimed the wording of Mrs May’s letter that Northern Ireland-only measures will be contained in the Withdrawal Agreement.
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.”
But Downing Street insisted Mrs May would never accept a proposal in which the UK is divided into two customs territories.
A 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.”