His comments came in a rare formal EU response to the U.K. queen's speech, in which the British government set out its domestic legislative agenda.

While the U.K. backed down from threats that it could introduce legislation to rip up the special Brexit deal covering Northern Ireland, it did make a veiled threat to Brussels that it could still act unilaterally. The document accompanying the speech — which was delivered by Prince Charles because of health issues for the monarch — said the U.K. would not allow negotiations with the EU to “stand in the way of protecting peace and stability in Northern Ireland.”

Boris Johnson's government also urged the EU to negotiate changes to the Northern Ireland Protocol, as the section of the Brexit deal is known.

The EU swiftly rejected that demand on Tuesday afternoon, arguing that the ball was in the U.K.'s court.

"From the very beginning, the EU has worked tirelessly to propose creative and durable solutions, showing flexibility on how the Protocol should be implemented," Šefčovič said in a statement. "It has shown that solutions can be found without changing the Protocol."

"We want to finalise these solutions as quickly as possible. But to do this, we need the UK government to show the same level of determination and creativity," the statement continued.