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 subject to some EU regulations, and requires checks on some 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, and Britain says it intends to extend the grace period unilaterally until October 1. The EU letter demands it reverse this decision.
Maros Sefcovic, the top EU official in charge of UK relations, has sent a separate letter to British counterpart David Frost, seeking talks in good faith to resolve the issue this month.
Britain says it has not violated the agreement.
Prime Minister Boris Johnson said on Monday the extension was simply a technical decision aimed at being fair. The protocol, he said, should guarantee trade between Britain and Northern Ireland, as well as across Northern Ireland's land border with Ireland.
"That's all we're trying to sort out with some temporary and technical measures which we think are very sensible, but obviously we'll look forward to our discussions with our EU friends and see where we get to," he said.