The European Commission has accused the UK of breaching EU rules by not putting forward a new commissioner.
The EU executive has asked the UK to nominate a member for the new Commission, due to take office next month.
But the UK has said it cannot put forward a name due to rules governing December's general election.
The move is the first stage in a process that could lead to a case at the EU's top court.
The EU has accepted an extension to the Brexit deadline, meaning the UK is now due to leave at the end of January 2020, if the Conservatives win the general election.
Incoming Commission President Ursula von der Leyen - who is replacing Jean-Claude Juncker - has asked the UK to nominate a commissioner, arguing it is obliged to do so under EU treaties.
But in a letter on Wednesday, the UK's EU ambassador said pre-election rules prevented ministers from putting forward a nominee.
Sir Tim Barrow is understood to have said guidance ahead of the 12 December poll prevented the government putting forward candidates for jobs at EU institutions.
But the Commission argues EU members cannot use domestic law to avoid compliance with EU treaty rules, which have a higher status.
The body has written to the UK government giving it until 22 November to respond with its view.
Commission team delay
The Commission says the unusually short timetable is "justified by the fact that the next Commission must enter into office as soon as possible".
The incoming Commission was due to take office on 1 November, but was held up after the European Parliament objected to several candidates.
It is now scheduled to assume power on 1 December, if the new team is approved by the European Parliament at a vote later this month.
The letter against the UK is the first stage in the Commission's infringement process.
This can eventually lead to the Commission taking governments to court at the European Court of Justice - although most cases are settled before then.
The court can force the country's authorities to comply, imposing financial penalties - either as a lump sum or as a daily payment as long as the infringement continues.
However, it can take many years for a case to move through the court.
The UK's current commissioner, Sir Julian King, has a role covering EU security policy.
The European Commission is responsible for proposing EU laws, implementing decisions and managing its day-to-day business.