Few developments in recent years have been as unambiguously positive as the dramatic improvement in relations between the United Kingdom and the Republic of Ireland. In 2011 I was privileged to accompany the Queen on her state visit to Dublin. It was four days I will never forget, producing an enthusiasm and outpouring of friendship most people never thought they would live to see.

But now we have a very big problem. The British and Irish governments seem to be on a collision course ahead of the crucial EU summit on December 14, with potentially disastrous consequences for the last chance to manage Brexit at all smoothly and for relations between the two countries. This month the EU, including Ireland, has started to argue that Northern Ireland should remain in the EU customs union and single market when the UK leaves, in order to maintain a completely open border. Doing so would draw a new economic border between Northern Ireland and the rest of the UK.

The solution to this requires understanding and patience on both sides of the Irish Sea, qualities that brought about the Good Friday Agreement and all the other gains of the last 20 years. In Britain, whether we voted Leave or Remain, we have to admit that our decision to leave the EU does create serious problems for Ireland. Brexit will deprive Ireland of her biggest ally in Europe on liberal trade policies and respect for national sovereignty on taxation. It is bound to throw up barriers to trading with and through us, their main trading partner and physical route to the Continent.