Published: 3:00 PM February 6, 2019 Updated: 6:02 PM September 17, 2020

Theresa May has been pressed to call a referendum on Irish unity as her Brexit backstop talks looked on the verge of collapse.

Sinn Fein president Mary Lou McDonald said the prime minister did not commit to saying in what circumstances an Irish unity poll might be called when she pressed her on it today.

The 1998 Belfast Agreement states that the secretary of state for Northern Ireland may call such a referendum if at any time it appears likely that the majority of voters in Northern Ireland would wish to leave the United Kingdom and enter into a united Ireland.

The exchange between the prime minister and Sinn Fein leader came during a series of meetings with political parties at Stormont House in Belfast.

McDonald said that a majority in Northern Ireland voted for the United Kingdom to remain in the European Union, and that by the region being taken out of the EU with the rest of the UK, the principle of consent - a key principle of the Belfast Agreement - has been damaged.

You may also want to watch:

'Nobody in Ireland has consented to Brexit, the people of the north of Ireland voted to remain,' she said.

'The Good Friday Agreement, the central premise of it as regards to constitutional arrangements, is the concept of consent. We believe that in attempting to coerce the north of Ireland out of the European Union against the democratically expressed wishes of the people, that the concept of consent has been damaged.

'We also know from polling data north and south that public sentiment is of a view that in the event of a crash Brexit, that people's preference would be for Irish unity.

'The only way that anyone can act in a democratic fashion is to respect consent, to respect the democratic wishes of the people of the north of Ireland and to use the Good Friday Agreement, its provisions, to guide us in the time ahead.

'So if there is a Tory crash, and Mrs May has conceded as much on the floor of the House of Commons, in that event, not alone will there be pressure for a referendum on Irish unity, there will be absolute democratic imperative to call such a referendum.'

When pressed over how May had responded to this during their meeting, McDonald said that neither the prime minister or secretary of state Karen Bradley committed to saying when or if an Irish unity poll would be called.

She said: 'But that is the secondary consideration, the primary issue here is the concept of consent which is hardwired into the Good Friday Agreement, the fact that the people of the north of Ireland have not consented to Brexit, the fact that business, farmers, academia, community sector, trade unions and political system almost unanimously regard Brexit as damaging and negative, almost unanimously regard the backstop as the absolutely bare minimum and the bottom line.

'The exception to that consensus is the DUP, the DUP cannot rule the roost on these matters, actually the people need to be in the driving seat, just as they voted against Brexit, in the event of a crash, I think the democratically correct thing to do, the compliant thing to do as regards the Good Friday Agreement, is to return to the people again, and in a structured way, to allow for the debate for the conversation for the transition and to put the question on Irish unity.

'Mrs May is a Unionist, she takes a different view.'