Northern Ireland’s largest nationalist party called on the Irish government Tuesday to prepare for the unification of the British region with EU-member Ireland, saying “bullish” Boris Johnson’s Brexit posed a threat.

Sinn Fein leader Mary Lou McDonald told supporters in Belfast that Johnson was “not my prime minister.”

Her comments come days after Irish Prime Minister Leo Varadkar said the question of the unification of Ireland and British-ruled Northern Ireland would inevitably arise if Britain leaves the European Union without a divorce deal on Oct. 31.

Johnson, who succeeded Theresa May as Britain’s prime minister last week, has vowed to take Britain out of the EU “no ifs or buts” on Oct. 31, deal or no deal.

“In the week since a jingoistic and bullish Boris Johnson strode into Downing Street and appointed a staunchly Brexiteer cabinet,” there has been speculation about what it means for the people of Northern Ireland, McDonald said.

“Elements of our political establishment tell us that now is not the time to discuss Irish unity. They are wrong. Now is exactly the right time to discuss it,” she said.

“The government has to begin planning for unity.”

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The possibility of a disorderly EU exit has encouraged those who want another referendum — on the reunification of Northern Ireland and Ireland.

While overall Britons voted 52 per cent in favour of leaving the EU in the 2016 referendum, 56 per cent of people in Northern Ireland voted to remain.

“The route back for the north into the EU is clear, Irish unity is the route back to the EU,” McDonald said.

Johnson, Varadkar finally speak

More than 3,600 people died in three decades of violence between Irish nationalists, who want a united Ireland, and the British security forces and pro-British “unionists” defending the region’s British status.

The 1998 Good Friday Agreement, which ended the violence, foresees the holding of referendums on both sides of the border on uniting the island, if London and Dublin see public support for that. The British government has said it does not believe there is sufficient support now.

McDonald said she would raise the issue of Irish unification at a meeting with Johnson in Belfast later this week.

The Sinn Fein leader also said interparty talks being held in Belfast in a bid to restore the power-sharing local administration, which collapsed 2½ years ago, were not making progress.

The 500-kilometre land border between Ireland and Northern Ireland has been seen as the biggest stumbling block for an orderly Brexit.  Johnson told Parliament last week he wanted to abolish the backstop, an insurance policy designed to prevent the return of border controls which were ended by the 1998 Good Friday peace agreement.

Irish Prime Minister Leo Varadkar on Tuesday reportedly told Boris Johnson, as have other European leaders, that the withdrawal agreement negotiated with his predecessor cannot be reopened. (Francisco Seco/Associated Press)

Varadkar called Johnson on Tuesday, nearly a week after his leadership win.

Varadkar said that the Brexit withdrawal agreement could not be reopened, and that “satisfactory” alternative arrangements have yet to be identified, according to the Irish government’s press office.

“Alternative arrangements could replace the backstop in the future, but thus far satisfactory options have yet to be identified and demonstrated,” Varadkar was quoted as saying.

Varadkar invited Johnson to Dublin for talks on Brexit and for a discussion of bilateral matters including Northern Ireland, the Good Friday Agreement and the Common Travel Area, the press office added.