Leaders from Ireland and Northern Ireland have made contradictory demands that threaten to hold up the Brexit negotiations.

• Leo Varadkar, the prime minister of Ireland, has threatened to veto progress in the talks unless a hard border with Northern Ireland is ruled out.
• Arlene Foster, the leader of the Democratic Unionist Party (DUP) of Northern Ireland, whose support Theresa May’s Conservatives need for their majority in Westminster, has said she will accept neither a barrier between the province and the rest of the United Kingdom nor an agreement that would force Northern Ireland to mirror EU regulations.

They can’t both have their way.

One or the other

To avoid closing the border, Northern Ireland would need to remain in the EU customs unions and possibly the single market — or at least write EU regulations into its own law, like Switzerland does.

If it doesn’t, then border controls would be needed to prevent smuggling and illegal immigration.

To avoid a border in Ulster, where the absence of one has helped keep the peace between Catholics and Protestants for twenty years, an alternative may be to introduce checks between Northern Ireland and the rest of the UK.

But that option has now been ruled out by the DUP as well.