Boris Johnson’s government appears to be preparing to nominate an EU commissioner, potentially breaching another Brexit promise after he failed to meet his “do or die” deadline to leave on 31 October.

The incoming president of the commission, Ursula von der Leyen, wrote to the prime minister last week, asking him to propose a British candidate or candidates for EU commissioner “rapidly, in the shortest time possible”.

Johnson’s spokesman said on Monday that the government would comply with the law. “The UK meets its legal obligations, and our officials remain in regular contact with the president-elect’s team,” the spokesman said.

That appeared to suggest that, as with the Benn act that obliged the prime minister to request a Brexit extension, Johnson would comply, albeit reluctantly and at the last minute.

In his first address to the House of Commons after becoming prime minister in July, Johnson told MPs that “under no circumstances” would he nominate a new commissioner – a promise aimed at signalling his determination to leave the EU on 31 October. He also promised to “unshackle” officials from EU meetings.

“Our national participation in the European Union is coming to an end, and that reality needs to be recognised by all parties. Indeed, today there are very many brilliant UK officials trapped in meeting after meeting in Brussels and Luxembourg, when their talents could be better deployed in preparing to pioneer new free trade deals or promoting a truly global Britain,” he said.

“I want to start unshackling our officials to undertake this new mission right away, so we will not nominate a UK commissioner for the new commission taking office on 1 December – under no circumstances – although clearly that is not intended to stop the EU appointing a new commission.”

With less than 12 hours until the lapse of an informal deadline for the UK to submit its nominee, Brussels said it had not yet received a letter from British authorities. “We are in close contact with them on this and indeed we are expecting an answer today,” said the chief spokesperson for European commission president-elect.

Von der Leyen hopes to take office on 1 December, pending approval of her team of commissioners in the European parliament. She needs to fill four EU commissioner vacancies, including the UK’s.

Under EU law, each member state must nominate a candidate or candidates to become European commissioner, a rule supported by Theresa May at her last EU summit in June. When Johnson was granted an extension last month, he also agreed that the UK would have “the obligation to suggest a candidate for appointment as a member of the commission”, as part of a promise not to undermine the workings of the EU.

However, lawyers for the EU institutions have concluded that even if the UK – or any other member state – failed to offer a candidate, Von der Leyen could still take office. But sources say she must ask the UK for a commissioner. “It’s not for us to decide not to have a UK commissioner. We have to follow the rules,” an EU source said.

Von der Leyen has encouraged the UK to propose female candidates in line with her wish to have a gender-balanced top team. That request is not binding, but is less favourable for the UK’s current EU commissioner, Sir Julian King. A career diplomat who now coordinates EU security policy, King took over in 2016 after the former Tory minister Jonathan Hill resigned following the Brexit vote.

The current standoff over a British nominee follows years of attempts to slim down the European commission by ending the requirement for each country to have its own commissioner. European commissioners are not meant to represent their countries and swear an oath to act independently and in the general interest of the EU.