Brussels has launched legal action against the UK over Boris Johnson‘s failure to nominate a new EU commissioner to send to Brussels.

In a statement the European Commission said it had issued “a formal letter of notice” to the British government ”for breaching its EU treaty obligations” and would be starting infringement proceedings.

Mr Johnson has so far refused to nominate a candidate for the incoming EU executive, stating that it is convention that such senior appointments cannot be made by the government during an election period.

On Thursday evening the EU rejected the explanation given by the UK, stating that “a member state may not invoke provisions prevailing in its domestic legal system to justify failure to observe obligations arising under union law”.

The move by the EU may turn out to be a political gift to the prime minister, allowing him to pick a fight with Brussels during an election campaign in which he needs to win over Brexit Party voters who take a dim view of the European institutions.

The government has been given a week to respond, with a deadline of 22 November.

Under EU treaties all 28 member states must have one commissioner, who collectively form the EU executive. The new commission wants to take office on 1 December this year, though it is likely to be delayed because of hold-ups in the approval procedure.

Because the UK has not left, under EU treaties it is required to have a commissioner. If Brexit happens according to the current plan the commissioner would serve just two months.

On Wednesday Ursula von der Leyen, the incoming commission president, wrote to Mr Johnson for the second time this month stating that the UK needed to name a candidate.

The UK government replied to the letter on Thursday, stating that it did not wish to frustrate the formation of the new EU executive but could not nominate a commissioner during the campaign period. The announcement of infringement proceedings followed on Thursday evening.

A UK government spokesperson said: “‘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.”

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In a statement, the commission said: ”As the guardian of the treaties, the European Commission has 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.

“The UK authorities have until 22 November at the latest to provide their views. This short time period is justified by the fact that the next commission must enter into office as soon as possible.”

The proceedings will take place under Article 258 of the Treaty on the Functioning of the European Union. After the UK has submitted a response or if none has been submitted, the commission can give a “reasoned opinion” on the situation.

Richard Corbett, the leader of Labour’s MEPs who is an influential authority on EU affairs, said: “In accepting the European Council decision of 29 October extending UK membership of the EU, Johnson accepted the continuation of the rights and obligations of membership, including the right and obligation to nominate a UK member of the European Commission.”