The EU is "negotiating with a partner it simply can't trust" in post-Brexit talks, Ireland's foreign minister has said.
On Wednesday, the UK said it would unilaterally extend grace periods for Irish Sea border checks, a move the EU said was a breach of international law.
Simon Coveney said he preferred "engagement", but the UK government was driving the EU towards legal action.
The grace periods mean procedures and checks are not yet fully applied.
Prime Minister Boris Johnson said the problems were technical and could be solved.
Northern Ireland has remained a part of the EU's single market for goods so products arriving from GB undergo EU import procedures.
The first of these periods will expire at the end of March, but the UK has said it will be extended until October.
Mr Coveney said progress was being made on the Northern Ireland Protocol and the timing of the UK's move could not be worse.
"That is why the EU is now looking at legal options and legal action which means a much more formalised and rigid negotiation process as opposed to a process of partnership where you try to solve the problems together," he told RTÉ's Morning Ireland programme.
However, speaking during a visit to Middlesbrough, the prime minister said: "We're taking some temporary technical measures to ensure there are no barriers in the Irish Sea, to make sure things flow freely from GB to NI and that's what you'd expect.
"But obviously these are matters for continuing intensive discussions with our friends and we'll continue to do that.
"I'm sure that with a bit of goodwill and common sense all these technical problems are eminently solvable."
The European Parliament has declined to set a date for its vote on the EU-UK trade deal in protest at what the EU sees as the UK's unilateral changes.
EU parliament group chiefs had been expected to set a date this month for its vote at a meeting on Thursday.
Conservative MP Simon Hoare, the chairman of the Northern Ireland Affairs Committee, said the UK needed to be alert to Mr Coveney's comments as it was important to maintain trust.
"If we go into these joint committees with an atmosphere of mistrust, my fear is... they won't be as fruitful as they could be and certainly the mood music is going to be in a minor key rather than in a major key," he told BBC Radio Foyle.
"You don't want to be moving house and then find in the first week that you move in that you're having a boundaries dispute with your neighbour."
Mr Coveney's party leader Leo Varadkar, who is Ireland's tánaiste (deputy prime minister), said the UK's actions were neither the way a friend, nor "a respectable, honourable country", should behave.
Northern Ireland Secretary Brandon Lewis said the UK was "taking several temporary operational steps to avoid disruptive cliff edges", and was continuing engagement with the EU through the Joint Committee, the body which facilitates ongoing negotiations between the EU and the UK.
"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," he said.
'Within the UK's remit'DUP MP Sir Jeffrey Donaldson said the EU's recent move to trigger Article 16 showed "very clearly that it remains within their remit to take unilateral action to protect the EU's single market".
"Under the Northern Ireland Protocol, the UK Government has the same power - to act unilaterally to protect the integrity of the UK internal market and trade between Northern Ireland and Great Britain," he told the BBC's Good Morning Ulster programme.
The EU reversed its Article 16 move within hours of taking the action, following condemnation from Belfast, London and Dublin.
European Commission Vice President Maroš Šefčovič later described the move as a mistake which had been made in the "intensity of the moment" of trying to secure Covid-19 vaccines.
The move by the UK government to unilaterally extend grace periods for Irish Sea border checks put Ireland in an uncomfortable position.
Mr Coveney said Ireland was lobbying at the EU level to obtain easements on trade between GB and Northern Ireland.
Other Irish MEPs have written to the European Commission calling for an extension.
But there will be other member states who will be reluctant to offer any concessions if the UK is going to ride rough-shod over the process.
So what could be the end game?
The two sides will continue negotiating. The EU says it is looking at legal avenues open to it under both the withdrawal agreement and the wider UK-EU agreement.
However, the UK may question what practical steps the EU will really take if grace periods are extended.
The UK statement on Wednesday indicated the extra time gained by an extension of the grace period could be used to develop systems to ease trade between GB and NI in the long term.
The UK was also at pains to say it is not walking away from the protocol, it is simply finding ways to implement it.
On Wednesday, Mr Šefčovič said the UK's move to extend grace periods amounted to "a violation of the relevant substantive provisions" of the NI Protocol.
He said the EU would respond in accordance with the "legal means" established by the protocol and the wider Brexit deal.
Following a call on Wednesday evening between Mr Šefčovič and Lord Frost, the Cabinet Office minister with responsibility for EU relations, a UK government spokesperson said "official-level notification" of the move was made to the commission earlier this week.
Lord Frost had underlined the extension was needed for "operational reasons" and such measures were "well precedented in other international trade arrangements, and that they were entirely consistent with our intention to discharge our obligations under the Protocol in good faith", said a government spokesman.
Further guidance would be provided later this week on a grace period for parcel movements from Britain to Northern Ireland, he added.
That is due to to end on 1 April, meaning all parcels would need customs declarations.