The European Union launched legal action on Monday against unilateral British changes to Northern Irish trading arrangements that Brussels says breach the Brexit divorce deal agreed with London.
The bloc 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.
Maros Sefcovic, the top EU official in charge of UK relations, has also sent a separate letter to his British counterpart, David Frost, calling for Britain to refrain from this measure, but also seeking talks on the issue.
The British government earlier this month unilaterally extended a grace period until October 1 for some checks on food imports to Northern Ireland. The period initially ran until the end of March.
The EU's executive European Commission promised to respond with the legal means established by the Brexit divorce deal and the trade agreement to what it said was Britain's second threat to breach international law.
Britain says it has not violated the protocol on Ireland/Northern Ireland.
Last September, Britain did acknowledge its Internal Market Bill would break international law by breaching parts of the Withdrawal Agreement treaty it signed in January 2020, when it formally left the EU. However, it dropped certain contentious clauses in December, two weeks before the two sides struck a trade deal.