British taxpayers face paying out for a large fine after the EU launched a legal action against Boris Johnson’s government over his failure to abide by the law and nominate a candidate for the new European commission.

Despite knowing for weeks that the UK would remain in the EU beyond 31 October, when a new EU executive had been due to be in place, Downing Street failed to put someone forward to join the bloc’s 28-strong top team.

Johnson instead belatedly claimed in a letter sent on Wednesday evening that he had been unable to make an international appointment due to purdah rules ahead of the general election on 12 December.

A spokeswoman for the European commission said the UK had breached its legal obligations despite the prime minister’s repeated claims in public that he would not defy the law.

The move could ultimately see the UK government dragged to answer for itself at the European court of justice, where judges have the power to issue large fines on member states that fail to live up to EU law.

The commission spokeswoman said the UK had until 22 November “at the latest to provide their views” on its formal infringement notice.

The incoming European commission president, Ursula von der Leyen, plans to have the EU executive team in place by 1 December.

In the commission’s response to the UK government’s letter, a spokeswoman said officials had “analysed this reply and considers that the UK is in breach of its EU treaty obligations”.

“The European commission recalls that, in accordance with established EU case law, a member state may not invoke provisions prevailing in its domestic legal system to justify failure to observe obligations arising under Union law”, the commission said.

For the European commission to be legally constituted, it needs all 28 member states to have a representative. A former ambassador to Ireland and France, Sir Julian King, is the current commissioner from the UK.

The move towards an infringement procedure by the commission suggests that, despite the earlier rejection of other nominations made by France, Hungary and Romania, 1 December remains a realisable target for the new commission to be up and running.

The formation of the new commission headed by the former German defence minister has already been delayed by a month owing to the European parliament’s rejection of nominees from three member states.

France’s commission candidate, Thierry Breton, was belatedly confirmed in his post as commissioner for the single market on Thursday despite only narrowly passing an examination of his financial declarations by the European parliament’s legal affairs committee earlier in the week.

Breton, a former chief executive at the software firm Atos, managed to assuage MEPs’ concerns during a grilling on Thursday over a potential conflict of interest between his links to tech companies and him being the next single market and industry commissioner.