Sinn Féin President Mary Lou McDonald has said that if there is a no-deal Brexit and hard border, the Prime Minister Boris Johnson must order an Irish border poll.
Speaking at an event in Belfast on Tuesday, she said the implications of Brexit for NI were enormous.
Her party has repeatedly called for a border poll.
She also called on the Irish government to appoint a Minister tasked with preparing for Irish reunification.
On Monday, the prime minister said there was “every chance” a Brexit deal with the EU could be struck, but the existing agreement with the EU has “got to go”.
But senior minister Michael Gove, who has been put in charge of preparing for no deal, has said the UK government was working on the assumption the UK would leave the EU without an agreement.
The prime minister is on a tour of the United Kingdom and is due to arrive in Northern Ireland this week.
Mrs McDonald said the party would meet Mr Johnson during his visit and remind him of “his obligations under the Good Friday and subsequent agreements including the provision for a unity referendum”.
She said: “If the British Government have factored into their calculations a hard border, then they must factor in a unity referendum as laid out in the Good Friday Agreement.”
She claimed that “Irish unity is the route back to the EU”.
During her speech she said that people from the Unionist community had discussed unity showing that there was a “growing discourse in the north and across the island,” on the matter.
“Peter Robinson, Paul Bew, Eileen Paisley, James Nesbitt, Harold Good and Mervyn Gibson – to name but a few – have all addressed the issue of unity from a distinctly unionist perspective,” she said.
“Yet political unionism has buried its head in the sand as the debate rushes around them,” she added.
She said that many people were “open” to discussing the prospect of a united Ireland.
“Fine Gael, Fianna Fáil and the SDLP should do the same,” she said.