The European Commission has launched "infringement proceedings" against Britain after Boris Johnson refused to nominate a new British EU commissioner.
In a statement, the commission said that it considered that the UK was "in breach of its EU treaty obligations".
It said that the Government had until November 22 to respond to the notice.
Mr Johnson has repeatedly said he would not appoint a new commissioner – even though all member states are legally obliged to do so.
Incoming commission president Ursula von der Leyen has said she wants to form a new College of Commissioners on December 1.
But in a letter to the commission on Wednesday, the Government said official pre-election guidance meant it could not make international appointments ahead of polling day on December 12.
Mr Johnson previous said he would rather be "dead in a ditch" than extend the UK's membership in the European Union, which he was forced to do after parliament declined to back a Brexit deal.