DUP leader Arlene Foster has ruled out backing the kind of customs compromise which could unlock Brexit negotiations and allow Boris Johnson to strike a new deal with the EU.
Foster insisted she could not accept any replacement of the Irish backstop which leaves Northern Ireland cut off from the rest of the UK.
But she did suggest the DUP could accept a time-limited backstop, the insurance policy to keep the border between Northern Ireland and the Republic of Ireland open, although conceded the EU was unlikely to offer this.
It comes amid speculation that the path to a deal could involve allowing Northern Ireland to stay in the EU customs territory, to maintain an invisible border without the backstop.
Johnson has already offered a concession to Brussels with an all-Ireland zone for trade in agri-food in a bid to break open talks on the backstop, which he has vowed to scrap.
This is not enough for the EU to agree it would keep the Irish border open but Foster suggested this was as far she would be willing to compromise if the DUP are to support any deal the PM brings back from Brussels.
She told a Policy Exchange fringe event at the Tory conference: “I think what some people don’t understand with the Democratic Unionist Party, when we set out a position, that is the position.
“Unfortunately, that was a misunderstanding which Theresa May had, she thought that we would change our position, but you have to understand the whole raison d’etre of the DUP is the union.”
She went on: “How can we possibly be in the EU customs union and also the UK?
“If you take that the way in which it was put in the withdrawal agreement, that we would be in a UK (NI) situation, think about what that would do to our agri-food for example.
“We would be different to the rest of Great Britain.”
Foster added: “Customs from a constitutional point of view, and from an economic point of view, do not work internally within the United Kingdom.”
On a time-limited backstop, she added: “Can I remind you what Leo Varadkar thinks of a time-limited backstop, he says it’s not a backstop at all?
“So in terms of the time limited backstop we have said in the past it’s something we would look at, I don’t think it’s something Leo Varadkar would look at.
“But certainly if a time-limited backstop was on offer it’s something we would look at, but I don’t believe it is at this moment in time.”
Although the DUP’s confidence and supply deal with the Tories may be seen as less important now that Johnson has lost his majority, the party can still unlock the support of hardline Brexiteeers for any deal, as it would not be credible for them to oppose a deal on unionist grounds if the Northern Irish party is satisfied.