The British prime minister, Theresa May, does not take time off from her responsibilities in London to travel to Scotland – even to address the conference of the Scottish Conservative party – without good reason. Of course, she has every reason since, unless she treads very carefully indeed, the United Kingdom’s threatened departure from the European Union could trigger a new Scottish independence referendum that could blow the 310-year-old Union apart.

However, following the dramatic results from the elections to the Northern Ireland Assembly last weekend. For the first time, losses suffered by the fiercely pro-UK Democratic Unionist Party (DUP) has left it with only a singleseat lead over Sinn Fein, the all-Ireland republican party, which wants an end to the partition of Ireland.

Even more significantly, in another historic first, the pro-UK Unionists of all parties no longer command a majority in the Assembly. The election result comes at a time of enormous significance not only for the future of Northern Ireland but of the imminent start of the Article 50 withdrawal negotiations between the UK and the EU.

As part of the 1998 “Good Friday” peace agreement which ended the latest phase of armed struggle between Irish republicans and the British state, the government of Northern Ireland has to be shared between the main pro-Union and the main Irish national parties. But the latest election was necessary because of the fall of the governing executive – run by the DUP and Sinn Fein in coalition – over a public heating supply scandal allegedly implicating the DUP leader and First Minister, Arlene Foster.

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Sinn Fein has said it will not join a government led by Ms Foster until a current inquiry into the scandal has exonerated her from any responsibility. The DUP will not agree to any other leader. The deadlock could now lead to the re-imposition of direct rule from London for Northern Ireland – something that both British and Irish ministers are desperate to avoid.

The politically explosive character of this crisis has been intensified as a result of the Brexit referendum. Although the UK as a whole voted narrowly to leave the EU, Northern Ireland (like Scotland) voted decisively to remain. Such a result was only possible because a significant number of Unionists joined Sinn Fein and other nationalist voters in backing continued EU membership.

The political temperature has been further increased because the British government has – so far – been unable to give any serious response to the question: “What happens to the border which divides Northern Ireland from the Republic of Ireland after Brexit?” More or less everybody (including ministers in London) want to leave the open border open, but in the case of London the qualification is “as far as possible.”

This seems, in practice, to come down to keeping customs control checks on the huge amount of daily trade movements across the border to a minimum. Dublin, Belfast and London know that a re-imposition of a ‘hard border’ would be a tempting invitation to dissident IRA factions to treat border customs and security institutions as a target for a renewed armed campaign.

But what of the free movement of people from the Irish Republic – which will of course remain an EU member– and Northern Ireland which could be outside the Union in, say, mid-2019? How can an open border be reconciled with the much-publicised boast of UK ministers that they will “Bring Back Control!” of immigration to the UK?

One possibility might be to introduce passport controls between Northern Ireland and mainland Britain even though both territories would be part of the same British state. This would be anathema to hardline Unionists and might even trigger unrest in that community as well as among Irish nationalists. Instead, the government may give up checking physical movements from the south to the north of Ireland and hope to pick up workers going into jobs on the British mainland via new social security checks.

One thing is clear: the question of the re-unification of Ireland is now back on the agenda. This is not only the demand among Sinn Fein and other Irish nationalists in Northern Ireland, it is being echoed by mainstream parties in the south. For the moment, a new referendum in Northern Ireland about ending the partition of the island seems unlikely – but it may in the end be unavoidable.

The Irish government is understandably alarmed by how things might get dangerously out of control as the hour of Brexit slowly approaches. Government ministers and officials have sent clear warnings to London that a ‘hard border’ across Ireland is not only economically disruptive but politically dangerous.

But if the Article 50 process leads to anything remotely resembling a “hard Brexit”, which ardently Eurosceptic, right-wing Conservative MPs would prefer, then tensions over the Irish border and pressure for the unification of Ireland would reach boiling point. May already knows that the loss of Scotland from the UK remains a real political risk.

For the British Tory government to ‘lose’ Scotland would look like real carelessness. If Northern Ireland was to leave the UK as well, the repercussions could trigger a fully blown British constitutional crisis – over and on top of the inevitable economic and social convulsions triggered by a final Brexit.