The Northern Ireland backstop remains the sticking point for May’s Brexit deal – the potential hiving off of a constituent nation of the UK into a different customs and regulations regimen with no assurance on when it might end. To avoid a hard border on the island of Ireland, Northern Ireland would still sign up to EU customs rules, with the rest of the UK, and effectively be in the Single Market until “alternative arrangements” are in place.

So why is this important? Why does it matter if produce coming over the Irish sea is subject to a few checks? Well, that’s because, historically speaking, customs unions are almost always a prelude to political integration.