Sinn Fein has told Prime Minister Boris Johnson a failure to hold a vote on Irish reunification in the event of a no-deal Brexit was “unthinkable”.
Party president Mary Lou McDonald and senior colleagues pressed the case for a border poll during their bilateral meeting with Mr Johnson at Stormont.
Under the terms of the Good Friday peace accord of 1998, the incumbent Northern Ireland Secretary must call a referendum on ending partition if there is evidence of a shift of public opinion in the region in favour of Irish unity.
Mrs McDonald said she told Mr Johnson that provision of the agreement must be triggered in the event of a no-deal Brexit.
“The way out of this for Ireland, if he is intent on crashing, if that’s the outcome, the way out for us democratically is to have a border poll and to allow Ireland and Irish people to decide our future,” she said.
Mrs McDonald accused Mr Johnson of being “complacent” about the damage she said the “Brexit nightmare” would inflict on the island of Ireland.
“His course of action, which seems to us that he has set the compass for a disorderly and a crash Brexit, we have challenged him very strongly on that policy,” she said.
“We set out very clearly that this would be catastrophic for the Irish economy, for Irish livelihoods, for our society, for our politics and for our peace accord.
“We made it clear to him that the extensive planning he tells us he is carrying out in respect of a potential crash Brexit has to include the constitutional question and the issue of a border poll here in Ireland.
“We have stated to him very clearly that Brexit in any event, but certainly a disorderly Brexit, represents in anybody’s language a dramatic change of circumstances on this island and it would be unthinkable in those circumstances that people would not be given the opportunity to decide on our future together.”
DUP leader Arlene Foster accused Taoiseach Leo Varadkar of engaging in “project fear”-style rhetoric over a united Ireland after Brexit.
Democratic Unionists accused him of breaching the 1998 Good Friday Agreement and said the conditions were not met for a border poll.
Leo Varadkar said it would be “provocative” for the Irish government to take steps to discuss a united Ireland at this stage but asserted in the event of a no-deal exit, more and more people in Northern Ireland will come to question the union.
Arlene Foster said: “It is project fear mark 2 from the Taoiseach.”
She claimed he had behaved “crassly” towards victims of the Northern Ireland conflict in the past.
“He needs to dial down the rhetoric, he needs to recognise the mandate of the Prime Minister and he needs to engage.
“He needs to get engaged and he needs to find a way forward.”
Prime Minister Boris Johnson told the Democratic Unionists a border poll was not something he was entertaining.
DUP deputy leader Nigel Dodds said: “Leo Varadkar is in breach of the Belfast Agreement by talking that up.
“The reality is that the conditions are not met and those who create this hysteria about it are actually running contrary to the agreement.
“Let’s get real here, the idea that many unionists will turn around and vote for a united Ireland on the basis of the current developments, I think is something that is not reflected in the reality of people on the ground.”