The EU on Friday urged the UK to adhere to a customs deal regarding Northern Ireland that was forged as part of the 2019 Brexit agreement.
EU lead Brexit negotiator Maros Sefcovic said there would be "serious consequences" if the UK suspends parts of the Northern Ireland Protocol.
What is the Northern Ireland Protocol?
The protocol allows the free flow of goods from the Republic of Ireland, which is part of the EU, to Northern Ireland. At the same time, goods now entering Northern Ireland from the rest of the UK are subject to customs checks.
A new customs border in the Irish Sea has angered pro-British unionists living in Northern Ireland. The EU has offered to reform the protocol and eliminate some checks on British goods flowing into Northern Ireland.
"This was a big move by us, but until today we have seen no move at all from the UK side," Sefcovic said in Brussels following discussions with UK Brexit Minister David Frost.
Frost said Friday that the protocol poses "fundamental difficulties" but added it could still be possibly reformed. At the same time, he said Article 16, which could suspend parts of the deal, is "very much on the table and has been since July."
By triggering Article 16, the UK would be forced to prove that the protocol has caused "serious economic, societal or environmental" difficulties.
If the UK invokes Article 16, the EU could slap targeted tariffs on certain British exports or even suspend a Brexit trade deal that was struck between the two sides last year. The most serious result could be a full-blown trade war between the UK and EU.
Watchdog: UK handling of Brexit borders 'successful'
The comments from the EU regarding Northern Ireland come after the UK's National Audit Office (NAO) said that the British government's handling of its new Brexit borders has been "largely successful."
At the same time, the spending watchdog said the UK's approach includes measures that are "not sustainable," with the NAO calling the Northern Ireland situation "inherently challenging."
"It is critical that UK government departments and devolved Northern Ireland departments deliver quickly on any agreement reached between the UK and EU and put in place the systems, infrastructure and resources required to make it work," the NAO said.
In addition to trade tensions over Northern Ireland, the UK and France are in a lengthy post-Brexit dispute over access to fishing waters in the English Channel.
wd/nm (AFP, dpa)