It is no surprise that David Davis, the Brexit Secretary, has refused to rule out a hard border on the Irish Sea.
That would mean travellers have to show a passport for travel between Great Britain and Northern Ireland.
It is an elementary feature of being a citizen of a country that you do not have to show your passport to travel within it. A person flying from Alaska to California, or from Corsica to Provence, does not have to show their passport. But someone flying from Belfast to Manchester have to do so.
This has been a risk for a long time, even before there was a referendum on withdrawal from the European Union.
It was proposed on security grounds in 2008 for all movements between the island of Ireland and Great Britain.
But what is alarming about the current uncertainty is the mind-set in London it betrays – always reluctant to do anything that might inflame Irish nationalist feeling, even if it might upset those people whose allegiance is to the UK.
No-one wants to see a hard land border between Northern Ireland and the Republic. But Britain seems set to exit the single currency, in which case it will probably leave the customs union, which could lead to customs checks at the land border.
Norway and Sweden’s long border, which has customs crossings points every 100 miles or so, shows how this can be done. Lorries have to use those points, but other vehicles can pass at smaller crossings where there are video cameras.
It is not, as Jim Allister and Ian Paisley and this column have observed on these pages, feasible for NI to stay in the single market if GB leaves it. That is a subtle step to Irish unity.
But the Common Travel Area and the fact that the UK and Ireland are both outside Schengen means that all entries to both islands are policed. Free travel between the islands is still feasible even if the jurisdictions have separate positions in relation to the single market. It can be combined with hi tech, low imprint customs posts at the border if the latter are needed.
Resolving this is one of the big challenges of Brexit, but unionists will not accept a hard border on the Irish Sea.