The move comes after Northern Ireland Secretary Brandon Lewis announced on March 3 that the UK planned to continue to bypass bureaucratic requirements on those imports until October 1.

A grace period on those checks is currently scheduled to expire at the end of March.

This three-month extension from the end of the Brexit transition period on January 1 was an agreed goodwill measure to allow the affected food industries to adapt to the new trade barriers across the Irish Sea.

A specific part of the Brexit withdrawal agreement, called the Northern Ireland Protocol, aims to eliminate the need for border controls between Northern Ireland (part of the UK) and the Republic of Ireland (an EU member).

The EU-UK trade deal, which went into effect at the start of 2021, demands customs checks on some goods arriving in Northern Ireland from the UK mainland, including food.

EU officials said they were blindsided by the UK's move to extend the grace period. Negotiators had been attempting to broke a compromise.

The EU's legal actions are twofold.

On Monday, the EU sent a letter to the UK to formally notify London of its alleged breach of the Withdrawal Agreement, a move that could potentially result in the UK being taken to an arbitration panel that may result in financial sanctions.

The formal notice "marks the beginning of a formal infringement process" as set out in the protocol, a senior EU official said, adding that the letter requests that the UK "carry out swift remedial actions to restore compliance with the terms of the protocol."

The second action -- another letter -- signals the deepening political fissure between the two parties, as it alleges that the UK has breached the provisions of the Withdrawal Agreement.

European Commission vice president Maroš Šefčovič sent the political letter to David Frost, co-chair of the joint committee, calling on the UK government to rectify and refrain from putting its proposed extension of the protocol into practice.

The letter called the UK's unilateral measures "a violation of the duty of good faith," and called on London to enter "bilateral consultations in the joint committee in good faith, in order to reach a mutually agreed solution as quickly as possible."

Stressing that those talks should begin by the end of March, the letter also issued a harsh rebuke of the UK's actions.

"... the UK must stop acting unilaterally and stop violating the rules it has signed up to," it added. "What we need in order to implement the protocol is mutual trust and this kind of unilateral action that we see from the UK, does not build trust."

The UK has denied breaching any protocol.

Earlier this month, UK Prime Minister Boris Johnson rebuffed claims of growing tensions between the bloc and the UK, saying he was sure "that with a bit of goodwill and common sense that all these technical problems are imminently solvable."

Yet one senior EU official said on Monday that the UK's move is "the second violation of international law on the same issue," referencing an October 2020 breach of the Brexit deal.

Irish Minister for Foreign Affairs Simon Coveney slammed the UK's extension of the grace period earlier this month, telling Irish national broadcaster RTÉ Radio on March 4 that the EU is "negotiating with a partner it simply can't trust."

The Northern Ireland Protocol has been a key point of contention throughout the Brexit talks, with Northern Ireland's Unionists, who are pro-British and pro-Brexit, opposed to the EU-UK trade deal that demands customs checks on some goods arriving in Northern Ireland from the UK mainland.

Northern Ireland First Minister Arlene Foster and her party, the Democratic Unionist Party, argue that the deal puts the future of union at risk.

Northern Ireland, plagued by a history of sectarian violence, is still divided by identity politics, with peacemakers concerned that the EU-UK spat might create a space for those grievances to breathe.