The DUP has stepped up warnings to British Prime Minister Theresa May not to compromise over the border on the island of Ireland in her efforts to secure a Brexit deal.
Following three days of talks with key figures in Brussels, DUP leader Arlene Foster said Mrs May could not in "good conscience" accept the proposals currently on the table from the European Union.
Her intervention came as Mrs May met key members of her cabinet in Downing Street to brief them on the progress in the Brexit negotiations.
The Prime Minister was reported to have played down the prospects of a breakthrough at next week's EU summit in Brussels, billed as the "moment of truth" by European Council president Donald Tusk.
In a statement Mrs Foster, whose party props up the British government at Westminster, said the EU plan would effectively mean imposing a trade barrier between the Northern Ireland and the rest of the UK.
"The Prime Minister is a unionist.
"Many of her cabinet colleagues have assured me of their unionism," she said.
"Therefore, they could not in good conscience recommend a deal which places a trade barrier on United Kingdom businesses moving goods from one part of the Kingdom to another."
Mrs Foster's latest shot across the bows came after the party had earlier made clear that it would be prepared to vote against the Budget and other domestic legislation if Mrs May crossed their "red lines".
Foreign Secretary Jeremy Hunt insisted that ministers would not sign up up to any plan which compromised the territorial integrity of the UK by imposing a "border in the Irish Sea".
"The DUP's red lines are actually Theresa May's red lines," he told BBC News.
"She has made it very, very clear that she will not allow there to be border down the Irish Sea, that the integrity of the United Kingdom must remain intact.
"I know that she will never sign up to a Brexit deal that compromises our territorial integrity."
However, Mrs Foster said the EU's proposals would place "an effective one-way turnstile" between Northern Ireland and the rest of the UK.
"Trade from Great Britain into Northern Ireland would be in danger of restriction. Indeed, Northern Ireland's access to any new United Kingdom trade deals would also be regulated by Brussels," she said.
Suspicions remain among hardline Tory Brexiteers that Mrs May is heading for a compromise which could tie the UK to EU customs arrangements indefinitely - something which Boris Johnson has warned would reduce Britain to a "permanent EU colony".
Negotiations between the two sides have focused on the proposals for a so-called "backstop" to ensure that there is no return to a "hard border" between Northern Ireland and the Republic after Britain has left the bloc.
However Brexiteers - including Brexit Secretary Dominic Raab - are insisting any arrangement which would see the UK effectively remain part of the customs union while negotiations over a free trade deal take place must be strictly time-limited - something the EU has been resisting.