Boris Johnson and Brussels are on an election campaign collision-course over whether the UK must send a new European Commissioner to Brussels.

Ursula von der Leyen, the European Commission's incoming president, on Wednesday wrote to Mr Johnson tell him has has to put forward a candidate to represent the UK on the EU's executive.

But Mr Johnson has long said he would not send a candidate, and has so far resisted despite other countries having send their picks.

Under EU treaties, each of the EU's 28 member states has to pick someone to join the EU's executive in Brussels, where they take on a particular portfolio.

The EU wants the commission to be finalised by 1 December, meaning any fracas over delayed caused by the UK would land squarely in the heat of the UK general election campaign.

The UK's current Commissioner is Julian King, who is in charge of security across the bloc.

The Brexit extension agreed by EU leaders in late October included an obligation to nominate a Commissioner, stating that the UK remained a member of the bloc past 31 October "with full rights and obligations... including the obligation to suggest a candidate for appointment as a member of the Commission".

A spokesperson for the European Commission said Ms Von der Leyen's letter encourages Mr Johnson to put forward a woman for the post to ensure gender balance in the College of Commissioners – effectively the EU's cabinet.

The request by the EU presents Mr Johnson with a dilemma because sending a commissioner would represent another broken promise, and could also annoy Brexiteers who might see it as a sign of continued EU influence.

But Downing Street might also relish an election-campaign bust-up with Brussels and use it to win over more of the Brexit Party voters who the Conservatives need to win a majority.

Lib Dem MEP Caroline Voaden said: "This is welcome news. Britain should be leading in Europe, not leaving. A UK Commissioner would ensure we are treated as equal partners as an EU Member State and fully represented in Ursula Von der Leyen's new Commission. Boris Johnson should propose a candidate capable of furthering the interests of the EU as a whole."

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Labour MEP Julie Ward meanwhile said: “Johnson refused to nominate a UK commissioner in August, and he failed yet again to deliver on his promises.