It is down to the controversial legislation which the UK government admits could break international law.
The Internal Market Bill is designed to protect trading within different parts of the UK, but Brussels says it breaches the Withdrawal Agreement struck last year.
European Commission President Ursula von der Leyen confirmed the news at a briefing earlier this morning.
"As you know we had invited our British friends to remove the problematic parts of their draft Internal Market Bill by the end of September.
"This draft bill is by its very nature a breach of the obligation of good faith laid down in the withdrawal agreement. Moreover, if adopted as is it will be in full contradiction of the protocol of Ireland and Northern Ireland.
"The deadline lapsed yesterday, the problematic provisions have not been removed, so therefore the Commission has decided this morning to send a letter of formal notice to the UK Government.
She said this is the first step in an infringement procedure from the EU Commission.
She added that "The letter invites the UK government to send its observations within a month and besides this the Commission will continue to work hard towards a full and timely implementation of the withdrawal agreement."
She concluded by saying that the Commission stands by their commitments.
