BRUSSELS—The European Union launched legal action on Monday against Britain for unilaterally changing trading arrangements for Northern Ireland that Brussels says breach the Brexit divorce deal agreed with London last year. The bloc has sent a letter of formal notice to kick-start an “infringement procedure”, which could lead to fines being imposed by the EU’s top court, although that could be at least a year off, leaving time for a solution to be found. Britain’s withdrawal agreement with the EU leaves the British-ruled province of Northern Ireland in the EU single market for goods and so requires checks on goods arriving there from other parts of the United Kingdom. Some of those checks are meant to start when a grace period expires at the end of March. The plans have already disrupted some supplies reaching supermarkets in Northern Ireland. Britain says it intends to extend the grace period unilaterally until …