Commission vice-president Maroš Šefcovic said in a statement that UK''s decision to unilaterally extend a grace period on checks for goods moving between Great Britain and Northern Ireland amounts to "a violation" of the so-called Northern Ireland Protocol.
The Protocol was designed to ensure an open border between Northern Ireland and the Irish Republic after Brexit.
"This is the second time that the UK government is set to breach international law," Šefcovic said. "This also constitutes a clear departure from the constructive approach that has prevailed up until now."
The EU''s anger was sparked by the British government's decision to extend until October a grace period for checks on agri-food entering Northern Ireland that was set to expire at the end of the month.
In September last year, the UK had already upset the 27-nation bloc when it considered — then backpedaled — legislation that would have given the Boris Johnson's government the power to override part of the Brexit withdrawal agreement relating to Northern Ireland.
The sensitivity of Northern Ireland's status was underscored earlier this year when the EU threatened to ban shipments of coronavirus vaccines to Northern Ireland as part of moves to shore up the bloc's supply. That would have drawn a hard border on the island of Ireland - exactly the scenario the Brexit deal was crafted to avoid.
Šefcovic held discussions Wednesday with cabinet minister David Frost, the former chief Brexit negotiator now responsible for EU relations.
"Lord Frost explained that the measures announced today, following official-level notification to the Commission earlier this week, were temporary technical steps, which largely continued measures already in place, to provide more time for businesses such as supermarkets and parcel operators to adapt to and implement the new requirements in the Protocol," a UK government spokesperson said.
Before their talks, Šefcovic said he would tell Frost "the European Commission will respond to these developments in accordance with the legal means established by the Withdrawal Agreement and the Trade and Cooperation Agreement."