The European Commission on Thursday sent the U.K. a letter of formal notice for breaching its obligations under the Withdrawal Agreement from the European Union. The move marks the beginning of a formal infringement process against the U.K., which has one month to reply to the letter.

The issue at the center of the conflict is the controversial Internal Markets Bill, over which U.K. special envoy Amal Clooney recently resigned.

The U.K.’s EU Withdrawal Agreement, finalized a year ago, states that goods travelling between Northern Ireland and the rest of the U.K. are subject to customs regulations after the end of the transition period. The Internal Markets Bill, if passed, will override the customs clause, thereby breaking international law.

“The U.K. government has failed to withdraw the contentious parts of the Bill, despite requests by the European Union,” reads a statement from the European Commission.

“The U.K. has breached its obligation to act in good faith, as set out in Article 5 of the Withdrawal Agreement,” the statement added. “Furthermore, it has launched a process, which — if the Bill is adopted — would impede the implementation of the Withdrawal Agreement. As a result, the Commission has launched infringement proceedings today in line with the provisions of the Withdrawal Agreement.”

Addressing the media on Thursday, European Commission president Ursula von der Leyen said: “As you know, we had invited our British friends to remove the problematic parts of their draft Internal Markets 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 adapted as is, it will be in full contradiction to the protocol of Ireland, Northern Ireland.

“The deadline lapsed yesterday, the problematic provisions have not been removed,” continued Von der Leyen. “Therefore this morning the Commission has decided to send the letter of formal notice to the U.K. government. This is the first step in an infringement procedure. The letter invites the U.K. 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. We stand by our commitment.”