The European Union's patience with the UK over the implementation of the Northern Ireland Protocol is "wearing very, very thin," the European Commission's Vice President said on Wednesday following a meeting with British negotiators.
Maros Sefcovic told reporters that London and Brussels are at a "crossed road" and reiterated threats that the bloc will act "swiftly, firmly and resolutely" if the UK does not respect the term of the Withdrawal Agreement.
He warned that Brussels could impose tariffs on British goods.
As part of the Brexit treaty agreed in December, and in order to avoid the creation of a physical border between the Republic of Ireland and Northern Ireland which many fear could spark sectarian violence on the island once more, the British province has so far remained in the bloc's customs unions.
This effectively created a border in the Irish Sea between Great Britain and Northern Ireland as certain goods now need to be checked before travelling across the sea.
Prime Minister Boris Johnson, who negotiated the Withdrawal Agreement now in force, has since called it "ludicrous". A grace period was initially agreed with London unilaterally decided earlier this year to waive checks.
The grace period is to expire on June 30 but Britain now wants to extend that waiver to 2023, which Brussels rejects.
Ahead of their meeting in London, David Frost, Britain's Brexit Minister, called on the EU to show "common sense" ahead of the meeting, arguing in an op-ed published in the Financial Times on Monday that the UK had presented the bloc with a "range of policy papers" to find a solution over the Irish issue.
Irish Foreign Minister Simon Coveney has refuted Frost's accusation that the EU is unwilling to negotiate, writing on Twitter earlier this week that "Lord Frost continues to lay blame for difficulty with Protocol at EU inflexibility. This is simply not the case. Maros Sefcovic & EU have consistently proposed new solutions.
Sefcovic also took aim at Frost's accusation, telling reporters that "EU has engaged creatively and tireless to find solutions" over the Northern Ireland Protocol and agreed to an initial grace period for border checks "because the UK was not technically ready to implement" it.
He said the 27-country bloc has demonstrated "a very constructive approach and enormous patience" but that the latter is now "wearing very, very thin".
"The UK has to abide by its legal obligations and perform these controls," he continued, highlighting "numerous and fundamental gaps in the UK's implementation of our agreement".
He added that the EU could take legal action, which would include arbitration and "cross retaliation" which could mean tariffs on certain goods.
Brussels already launched legal action against London over its decision to unilaterally extend the initial grace period. Sefcovic said that as things stand, the case could make it in front of the European Court of Justice in the autumn.