But she won't do that, says DUP deputy leader
Prime Minister Theresa May will "rue the day" she calls the DUP's bluff, the party's deputy leader Nigel Dodds warned if any Brexit deal brokered with the EU creates an apparent customs border down the Irish Sea.
The MP said his party would have "no hesitation" in bringing down the government.
"We are not bluffing," he said.
He was responding to reports of a leak of the government's latest plan to try and solve the Irish question in the Brexit talks. The radical plan would see Northern Ireland have joint EU/UK status with a special buffer zone around the border to allow traders to operate under the Republic's rules thereby avoiding checks. The plan was roundly dismissed.
Speaking to the BBC, Nigel Dodds was asked what the party would do if the final EU deal effectively created a border in the Irish sea.
He said: "The Prime minister has made it clear and the government has made it clear even today that they can not and will not accept a customs border down the Irish sea. That they will maintain the integrity of the UK's common market and the constitutional integrity of the country.
"I have no doubt about the Prime Minister's commitment to that. Nothing will be implemented that will breach those clearly stated positions."
The North Belfast MP said they didn't "envisage being faced with any kind of dilemma, we don't envisage being put into a position of bringing down the government" should the Prime Minister appear to agree to a sea border.
"We have made it very clear we would have no hesitation.. anything that would diminish the Union of the United Kingdom would be a clear red line for us."
Put to him the Prime Minister could call the party's bluff, Mr Dodds responded: "She may do and she will find that we are not bluffing.
"She will find very clearly that it isn't a bluff and she will rue the day that she did that if she decided to go down that path
"But I have no belief whatsoever that she will."
Mr Dodds, the DUP leader at Westminster, said his party worked regularly with the government on a wide range of issues and had held discussions on Friday. Although he would not be drawn on if the leak of the new plans was brought up.
"We will continue to work with the government to get the best deal for Northern Ireland," he continued.
"Because there are areas such as the single electricity market and in terms of animal health regulation where we already have special arrangements for the island of Ireland.
"That's not in any diminution of the red lines we have put down.
"What we need to be very, very clear about is there are no added barriers either through single market regulation or customs which prevents or hinders the economic trade that goes on between Northern Ireland and the rest of the UK."
He added: "We want the best deal for Northern Ireland. What we have always been concerned about has been that if we focus to a disproportionate amount on keeping the border open between Northern Ireland and the Irish Republic - and we do want it to be absolutely frictionless and we don't want a hard border - but that can't be at the expense of erecting any kind of barrier or hinders to trade between Northern Ireland and the rest of the UK which is by far our biggest market.
"That is our concern - we want to see balance in all of this and ensure our best interests are protected."