Dangerous as the situation is, there is something faintly absurd about the appearance of UK and then French naval vessels off the Jersey coast amid a dispute over fishing rights. Britain sent its ships after dozens of French fishing boats set sail from Normandy ports to St Helier, and a reckless French threat to cut electricity supplies through undersea cables to the Channel island. The episode is both a warning of how quickly minor discords can escalate in the post-Brexit era, and a case study in how not to resolve them.

There are, in reality, two intersecting disputes. One is over the conditions the Jersey authorities have attached to licences to continue fishing off the island, a UK crown dependency, including demands that French ships be equipped with monitoring devices. Britain says under its EU trade agreement, French boats must show evidence of their previous activity in Jersey waters. French fishing crews are also complaining more broadly about difficulties in receiving licences to fish in British coastal waters.

Politics are at play too. Xavier Bertrand, leader of the northern Hauts-de-France region which borders Normandy, is a potential challenger to Emmanuel Macron in next year’s French presidential election. Annick Girardin, the maritime minister who threatened to pull the plug on Jersey, is the daughter of a Breton fisherman.

EU states and institutions have a political interest in showing Brexit is an unattractive model, and have sometimes seized opportunities to do so. UK hauliers complain of unnecessary zeal by French authorities in applying EU rules, with stories of shipments returned because paperwork was signed with the wrong colour pen. The European Commission has just recommended against readmitting the UK to the Lugano convention, which ensures civil and commercial court judgments are enforceable across the EU, though several states had voiced support.

For its part, Boris Johnson’s UK government insisted the fisheries sector would be a beneficiary of Brexit, then signed a deal that left it, for now, worse off. Despite Whitehall denials, it may not be entirely coincidental that naval ships appeared in the Channel on the day of local elections in England.

Yet the fishing spat risks spilling into other areas. It comes just as the European parliament’s ratification of the UK-EU trade deal last week was supposed to bring into full operation two dozen committees and working groups to manage relations. It shows how far the UK is from finding an equilibrium with the EU as it tries to reach agreement in other key areas of regulatory co-operation — from linking the UK’s emissions trading system with the EU’s to agreeing on future regulations on chemicals.

The Johnson government has complicated matters by putting Lord David Frost, the hardliner who negotiated the bare-bones trade deal, in charge of EU ties. Trust has been badly dented by the UK’s decision to take unilateral steps to ease the impact of Brexit on Northern Irish businesses, over which Brussels has launched legal action. For the EU, reaching agreement on implementing the Northern Ireland “protocol” has become a test of UK good faith. One way to ease that situation would be to reach a veterinary agreement that could sharply reduce checks on goods of animal origin moving into the EU.

Resolution of the dispute over the status of the EU’s ambassador to London this week suggests at least some in Westminster seek a grown-up relationship. But all sides need to move from posturing to pragmatism, and develop a habit of resolving disputes not through gunboat diplomacy, but in a manner befitting mature democracies.

This editorial has been amended to make clear Normandy borders the Hauts-de-France region and is not a part of it.