The European Commission has said the UK is in violation of its post-Brexit obligations after unilaterally deciding to continue Irish Sea border grace periods until October.
Britain is set to breach international law for a second time, vice-president Maros Sefcovic warned.
The Irish Government also branded the intervention extending the soft-touch regulatory regime on some goods entering Northern Ireland from Great Britain as “deeply unhelpful”.
A commission statement on behalf of Mr Sefcovic said: “Following the UK Government’s statement today, vice-president Sefcovic has expressed the EU’s strong concerns over the UK’s unilateral action, as this amounts to a violation of the relevant substantive provisions of the protocol on Ireland/Northern Ireland and the good faith obligation under the Withdrawal Agreement.
“This is the second time that the UK Government is set to breach international law.
“This also constitutes a clear departure from the constructive approach that has prevailed up until now, thereby undermining both the work of the Joint Committee and the mutual trust necessary for solution-oriented co-operation.”
The Northern Ireland Protocol is designed to prevent the imposition of a hard border on the island of Ireland by keeping Northern Ireland following EU trade rules.
It has caused disruption to some goods travelling from the rest of the UK after suppliers encountered extra red tape.
Businesses in Northern Ireland have been pressing for an extension to the grace periods to avoid a cliff-edge plunge into extra bureaucracy linked to the protocol.
The British Government has said grace periods like those for supermarket agri-food movements from the rest of the UK to Northern Ireland “will continue” until October.
Northern Ireland Secretary Brandon Lewis said: “As part of the pragmatic and proportionate implementation of the Northern Ireland Protocol, the Government is taking several temporary operational steps to avoid disruptive cliff edges as engagement with the EU continues through the Joint Committee.
“These recognise that appropriate time must be provided for businesses to implement new requirements, and support the effective flow of goods between Great Britain and Northern Ireland.”
Ministers face a backlash from unionists who fear the protocol threatens Northern Ireland’s place in the UK internal market.
Mr Lewis said: “For supermarkets and their suppliers, as part of the operational plan the UK committed to at the UK-EU Joint Committee on February 24, the current Scheme for Temporary Agri-food Movements to Northern Ireland (STAMNI) will continue until October 1.
“Certification requirements will then be introduced in phases alongside the rollout of the Digital Assistance Scheme.”
He said further guidance will be provided later this week on parcel movements from Great Britain to Northern Ireland, to provide necessary additional time for traders beyond April 1.
“Guidance will also be set out to help address practical problems on soil attached to the movement of plants, seeds, bulbs, vegetables and agricultural machinery.”
The first of the grace periods had been due to expire at the end of March.
Supermarkets would have had to produce export health certificates for all shipments of animal products.
Irish Foreign Affairs Minister Simon Coveney said unilaterally continuing the grace period undermines the UK’s commitment to the protocol.
He said: “A unilateral announcement is deeply unhelpful to building the relationship of trust and partnership that is central to the implementation of the protocol.”
Mr Coveney met Mr Lewis and the Cabinet Office minister responsible for EU-UK relations, Lord Frost.
He said: “I made clear to them my regret that the UK had moved in a unilateral way, rather than working in continued partnership with the EU in accordance with the EU-UK joint statements of 11 and 24 February.”
The DUP is aiming to undermine the protocol.
The party’s Westminster leader, Sir Jeffrey Donaldson, said: “Whilst supermarkets and those bringing in goods via our ports from Great Britain will be relieved to see extensions to the grace periods, we will be continuing to press the Government for a permanent solution.
“Grace periods do not provide the long-term certainty that businesses and consumers in Northern Ireland require.
“The protocol has been demonstrated to be unworkable.”
The DUP’s Stormont Agriculture Minister, Gordon Lyons, sparked controversy on Friday night when he announced he had ordered officials to halt work on planned permanent facilities to carry out inspections on goods from Great Britain.
Sinn Fein president Mary-Lou McDonald said: “At last week’s meeting of the Joint Committee, Michael Gove and Maros Sefcovic reaffirmed support for the Irish Protocol and the need to work together to deal with issues that have arisen.
“It is incredible that one week later the British government has gone on a solo run and taken unilateral action.
“This was completely unnecessary, totally undermines the work of the Joint Committee and puts it on an immediate collision course with the European Union.”