The EU has begun legal proceedings against the UK over its plans to override sections of the Brexit divorce deal it signed in January.
It comes after an EU request for the government to remove controversial sections of the Internal Market Bill passed on Wednesday.
The “letter of formal notice” could lead to a court case against the government at the EU’s top court.
It comes as trade talks between the two continue in Brussels.
European Commission President Ursula von der Leyen said the UK would have a month to respond to the EU’s concerns over the draft legislation.
She added that the bill was a “full contradiction” of previous UK commitments over how a hard border on the island of Ireland should be avoided.
The government has insisted the bill is a necessary “safety net” to protect trade between different parts of the UK.