Sinn Fein has said a failure to hold a vote on Irish reunification in the event of a no-deal Brexit would be “unthinkable”.
Party president Mary Lou McDonald said she pushed Boris Johnson for a referendum during the Prime Minister’s visit to the Northern Irish parliament on Wednesday.
“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.
She accused Mr Johnson of being “complacent” about the damage leaving without a deal would cause for all of Ireland.
“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,” she said.
“It would be unthinkable in those circumstances that people would not be given the opportunity to decide on our future together.”
However she acknowledged the Prime Minister “obviously takes a different view on Brexit generally and the issue of the Union”.
Northern Ireland has been without a devolved government for two-and-a-half years due to a collapsed powersharing deal between Sinn Fein and the DUP.
Mr Johnson said he was in Belfast to get the government “up and running again”.
“My prime focus this morning is to do everything I can to help that get up and running again because I think that's profoundly in the interests of people here, of all the citizens here in Northern Ireland,” he said.