The DUP has welcomed the Government's decision to unilaterally extend Irish Sea border grace periods until the autumn, but Sinn Fein has branded it an "unnecessary solo run".
Irish Foreign Minister Simon Coveney described the move as "deeply unhelpful", but Ulster Unionist leader Steve Aiken accused him of "showing gravity-defying levels of arrogance".
The EU claimed the UK was breaking post-Brexit obligations and was set to breach international law for a second time.
The row followed an announcement by Secretary of State Brandon Lewis that the grace period for supermarket agri-food movements from Britain to Northern Ireland would continue until October.
Certification requirements will be introduced in phases alongside the rollout of a digital assistance scheme, he added.
Local businesses have been pressing for an extension to avoid a cliff-edge plunge into extra bureaucracy linked to the protocol. The first of the grace periods was due to expire at the end of March, when supermarkets would have had to produce export health certificates for all shipments of animal products.
DUP 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 Government clearly recognises the economic problems, but it must also acknowledge the political rejection of the protocol."
Sinn Fein president Mary Lou McDonald said: "At last week's meeting of the Joint Committee, Michael Gove and Maros Sefcovic re-affirmed 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 that 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 EU." Mr Coveney said: "A unilateral announcement is deeply unhelpful to building the relationship of trust and partnership that is central to the implementation of the protocol."
European Commission vice-president Maros Sefcovic expressed serious concern over the Government's action, which he said was "a violation of the relevant substantive provisions of the protocol on Ireland/Northern Ireland and the good faith obligation under the withdrawal agreement".
Mr Sefcovic added: "This is the second time that the UK Government is set to breach international law."
But the Ulster Unionists were scathing of both Dublin and Brussels. Mr Aiken said: "The fact that the EU and Irish government have reacted in such a dramatic, over-the-top manner to what are minimal changes to help protect businesses in Northern Ireland tells you all you need to know about their real intentions.
"Simon Coveney and his EU counterparts are showing gravity-defying levels of arrogance given their past actions.
"If we moved at the snail's pace which the EU is moving at, businesses would be pulling their shutters down."
But Alliance deputy leader Stephen Farry MP said: "While this announcement from the UK Government may be superficially attractive in the short-term, it may bring long-term consequences. The legal basis under the Withdrawal Agreement for such unilateral steps is at best unclear. One set of unilateral moves may invite other such steps."
On Wednesday night, Cabinet member Lord David Frost said the UK's intervention should allow time for constructive discussions with counterparts in Brussels. A Government spokesperson said: "Lord Frost explained that the measures announced today, following official-level notification to the Commission earlier this week, were temporary technical steps, which largely continued measures already in place, to provide more time for businesses such as supermarkets and parcel operators to adapt to and implement the new requirements in the Protocol."