The EU is responding to the United Kingdom's unilateral decision to extend a grace period allowing exports from the rest of Britain to Northern Ireland largely without checks. The checks were part of the Brexit accord.
The European Union is taking legal action against the UK following its unilateral decision to ease the terms of the Northern Ireland Protocol, the bloc announced on Monday.
The new checks in the Irish Sea between Britain and Northern Ireland causing trade disruption, and their subsequent removal by Boris Johnson's government, have increased tensions between the EU and the United Kingdom.
The European Commission has issued a letter to the British government as a formal notice that it had breach EU law, which could cause the European Court of Justice to impose a fine on the UK.
Earlier in March, the United Kingdom had decided to extend a grace period on new rules for exports from the British mainland to Northern Ireland without seeking the EU's approval.
The two sides fell out over the issue last year, when British ministers re-wrote UK law linked to the Brexit divorce treaty that would have given them powers to ditch large parts of the agreement relating to the province.
A senior European Commission source, speaking on the condition of anonymity, accused the UK of "a second violation of international law on the same issue."
"This undermines trust and undermines an understanding that we can build a trustful relationship," the official added.
The source said the EU executive has made clear that it would be "willing to show further flexibility on this issue."
"And yet, the response of the UK government is to take unilateral action."
But a British government source said that "all sides need to keep in mind the fact that the [Northern Ireland] Protocol depends on cross-community consent and confidence if it is to work."
They added that it is not "something that should warrant legal action."
The White House waded into the row later on Monday, telling Britain and the European Union to preserve the Good Friday Agreement that Washington helped broker in 1998.
"We continue to encourage both the European Union and the UK government to prioritise pragmatic solutions to safeguard and advance the hard won peace in Northern Ireland," said White House spokeswoman Jen Psaki.