In an apparent reversal of a position she took on Monday, Sinn Féin leader Mary Lou McDonald yesterday said there would have to be Border poll if the UK crashed out of the EU next year without an agreement.

On Monday, in an interview with the Press Association, Ms McDonald said the Border poll question should be put to one side until the “dangers” posed by Brexit are mitigated.

Ms McDonald said she disagreed with those who argued that a hard or chaotic Brexit should be the trigger for a referendum on unification.

The Sinn Féin president insisted that would be the wrong climate for a substantive debate on the constitutional issue, PA reported.

“The obvious thing would be to say ‘well have the Border poll and remove the Border, if the Border is the problem, simply take it away’ and there is a certain logic to that,” she said.

“I am very, very conscious that you can’t come at this issue in that simplistic way.

‘Irish unity’ “I would prefer, it is my strong preference, that we have sequencing that firstly delivers a level of economic and social certainty, in as much we can be certain, and stability and, from that base, we then continue the conversation about Irish unity,” she told PA.

However, speaking to journalists at Leinster House yesterday, Ms McDonald said that if there was to be a hard Brexit next year, the British government would have to hold a Border poll.

“Should the British government insist on crashing the North out of the European Union, well then be very clear that they would have no option but to put the constitutional question. I mean it would be an absolute imperative in those circumstances,” Ms McDonald said.

“Let me emphasise that’s not the ideal scenario as far as I’m concerned . . . [but] should there be a crash and should there be the consequences of a crash - the hardening of the Border, the undermining of the Good Friday agreement, robbing citizens in the North of basic rights, of course the constitutional question would have to be put,” she said.

“No one should imagine that Theresa May’s government could crash the North, against the democratic wishes of of the people, out of the European Union, cause all of that jeopardy and all of that damage and not put the constitutional question. That would be absolutely a non-runner.”