• "The European Commission will respond to these developments in accordance with the legal means established," the statement also said.
• This is not the first time that Brussels and London are at odds over their post-Brexit arrangements.

LONDON — The European Union is threatening new legal action against the U.K. over differences on their post-Brexit trading arrangements.

As part of its departure from the EU, the U.K. agreed to conduct checks on goods moving across the Irish Sea, going from Scotland, Wales and England to Northern Ireland. The latter has remained part of the EU's single market for goods to avoid a hard border with the Republic of Ireland in what's known as the Northern Ireland Protocol.

The U.K. had until the end of this month to put forward these checks, but it has decided to extend the implementation period until October. A move that the European Commission, the executive arm of the EU, said breaches their agreement and therefore international law.

Maros Sefcovic, vice-president of European Commission, "expressed the EU's strong concerns over the U.K.'s unilateral action, as this amounts to a violation of the relevant substantive provisions of the Protocol on Ireland/Northern Ireland," the commission said in a statement on Wednesday before a call between the EU's and the U.K.'s representatives.

"The European Commission will respond to these developments in accordance with the legal means established," the statement also said.

The U.K. government has said that it informed the commission "earlier this week" before making the announcement public and that extending the grace period for implementation is a "temporary" technical step "to provide more time for businesses such as supermarkets and parcel operators to adapt to and implement the new requirements."

Supermarkets and other food retailers will need health certificates when shipping animal products.

Simon Coveney, Ireland's foreign affairs chief, said in a statement that the U.K.'s decision was "deeply unhelpful to building the relationship of trust and partnership that is central to the implementation of the Protocol."

"The Irish Government's focus remains on ensuring that the Protocol, as an international agreement concluded by the EU and UK, is fully implemented. It is the agreed solution to the problems created on the island of Ireland by Brexit," he added.

Coveney said he expressed his regret over the move during a meeting with Northern Ireland Secretary Brandon Lewis and Lord Forst, the Cabinet Office minister responsible for EU-U.K. relations.

This is not the first time that Brussels and London are at odds over their post-Brexit arrangements.

In October of last year, the EU started legal proceedings against the U.K. after the government put forward a bill that would have overridden the same deal over Northern Ireland.

In the end, after several weeks of meetings and discussions, the U.K. decided to drop the controversial clauses in the bill, which paved the way for a trade agreement to be agreed upon on Dec. 24.