The Prime Minister must call a border poll if there's a hard border on the island of Ireland post-Brexit, the Sinn Féin President has said.
Mary Lou McDonald was addressing party colleagues and members in Belfast where she said the backstop is the 'bare minimum' her party will accept as an outcome.
However, Boris Johnson, who took office just last week says he wants to scrap the current withdrawal agreement of which the backstop proposals are part of.
The backstop has been designed to maintain seamless trade across the border between Northern Ireland and the Republic of Ireland if a conclusive deal can not be reached between the UK and EU.
It would also be in place until a deal is reached between both sides.
However, PM Johnson and the DUP are totally opposed to it saying it splits Northern Ireland off from the rest of the UK politically and potentially economically.
Sinn Féin's position on the backstop has not changed since it first emerged from former Prime Minister Theresa May's deal, when it was first struck with the EU.
Speaking to an audience which included ex-President of the party, Gerry Adams, Ms McDonald 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.
"The route back for the North into the EU is clear. Irish unity is the route back to the EU.
"We will be meeting Boris Johnson in the coming days. We will remind him not only of the need to respect the wishes of the majority to remain within the EU.
"We will also remind him of his obligations under the Good Friday and subsequent agreements including the provision for a unity referendum."
She added that she believes 'partition has failed' and that the DUP were more concerned about protecting the union over anything else.
The Sinn Féin President also gave an update on the talks to revive the power-sharing government at Stormont which has been collapsed since January 2017.
She said meetings "have been held but little progress was made".
"This round of talks to restore the power-sharing government have thus far failed to achieve agreement," she said.
"This is because these key issues have not been faced up to yet."
But DUP MLA Christopher Stalford says Sinn Féin are 'bereft of ideas' adding that Northern Ireland would have to make 'the best of the situation' in the case of a no-deal scenario.
In the case of dealing with a no-deal exit, he said: "One of the things that can do that is to ensure the restoration of devolved government at Stormont.
"For more than two years now, the people of Northern Ireland have been left in a kind of limbo because Sinn Féin refuse to fulfill their obligations and serve the people in the Assembly. That's what I think we should be doing.
"Not engaging in hyperbole or megaphone diplomacy directed at the Prime Minister."