The EU has begun legal proceedings against the UK after it refused to ditch plans to override sections of its Brexit divorce deal.
An EU deadline for the government to remove sections of the Internal Market Bill passed on Wednesday.
The “letter of formal notice” could eventually lead to a court case against the UK at the European Court of Justice, the EU’s top court.
The UK said it would respond to the letter “in due course”.
However, the EU will continue talks over a post-Brexit trade deal in Brussels. Prime Minister Boris Johnson has said both sides should “move on” if a deal was not reached by mid-October.
In a brief statement, Mrs von der Leyen said the bill was a “full contradiction” of previous UK commitments over how a hard border on the island of Ireland should be avoided.
MPs gave their final backing to the bill earlier this week. However, it will have to be approved by the House of Lords before it becomes law.
In a bid to head off a potential rebellion from Tory backbenchers, ministers have granted the Commons a say before powers to override the Brexit divorce deal could be used.
The letter sent to the UK is the first stage in the process the Commission uses against countries it believes have broken EU law.
It can end with the Commission taking governments to court at the European Court of Justice.
However, most cases are settled before then – and it can take many years for a case to move through the court.