The EU on Thursday launched a legal case against London for failing to nominate a commissioner, despite the Brexit delay that keeps Britain in the bloc.
The European Commission, the EU's executive arm "has today sent a letter of formal notice to the United Kingdom for breaching its EU treaty obligations by not suggesting a candidate for the post of EU Commissioner," a statement said.
The move, known in EU jargon as an infringement procedure, begins a long process involving several warnings, but that could eventually lead to suing Britain at the EU's highest court to force compliance.
Ursula von der Leyen, the incoming president of the Commission, had written twice to Prime Minister Boris Johnson urging him to nominate a commissioner.
The former German defence minister had hoped to head a 28-strong team of top officials from each of the EU member states in time for her commission to take office on December 1.
But London notified the EU on Wednesday after much delay it will not nominate a British member to her top team before the December 12 general election.
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"We have written to the EU to confirm that pre-election guidance states the UK should not normally make nominations for international appointments during this period," a UK official said Thursday.
Britain plans to leave the bloc on January 31 and Johnson is fighting an election campaign on a pro-Brexit ticket.
Von der Leyen's November 1 target date for taking office has already come and gone, after MEPs rejected her French, Hungarian and Romanian nominees.