Once Britain leaves the European Union, the only land border it will have with the bloc (besides Gibraltar) will be the 310-mile open border between Northern Ireland and the Republic of Ireland. Brussels would not be able to accept people crossing into (and out of) newly non-EU territory without being policed, so can they be satisfied while keeping travel flowing?

The British acknowledge that both sides will need to "show flexibility and imagination" in order to avoid "a return to the border posts of the past". They have started the ball rolling by laying out how they want to resolve it in a new paper (something the Europeans have yet to do). Britain envisages an "invisible" border between Ireland and Northern Ireland without "any physical border infrastructure" and "light touch" technology handling any checks. The EU claims that it too wants to avoid a hard border, but it is only possible due to the bloc's ideological rigidity.

Brussels has tried to shift the blame preemptively onto Britain. Ireland's EU commissioner Phil Hogan warned earlier this week that the island could be the "biggest victim of this mess" and accused the UK of "high-level delusion". But the Republic of Ireland's then foreign minister Charlie Flanagan admitted last year that the EU would be "ultimately" responsible, rather than the United Kingdom, for imposing any hard border.