The Minister for Foreign Affairs has said the EU is negotiating with a partner it simply cannot trust following yesterday's move by the British government to unilaterally change how the Northern Ireland Protocol is implemented.
Speaking on RTÉ's Morning Ireland, Simon Coveney said the difficulty is that the British government has changed its approach.
He said progress was being made on the protocol and the time of the UK 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."
Mr Coveney said he does not favour legal action but favours engagement based on trust between both sides.
"If the UK cannot simply be trusted because they take unilateral action in an unexpected way without negotiation, well then the British government leaves the EU with no option and that is not where we want to be," he said.
The UK move is really unwelcome and it is the "British government essentially breaking the protocol" and its own commitments again, he added.
The EU is now having to consider how to respond to that, Mr Coveney said.
The British decision to act unilaterally is "clearly in breach of the protocol" and the commitments that were made a few weeks ago, he added.
Menwhile, former Taoiseach Bertie Ahern has called for calm from both sides of the negotiations.
Mr Ahern, who was involved in negotiations which resulted in the Good Friday Agreement, told Drivetime there should be no retaliation by the EU.
"I would love to see people calm it down and see if people can sit down and negotiate what are very technical issues, but not to be doing it in an arbitrary way like they did yesterday," he said.
Mr Ahern said the decision to unilaterally break the withdrawal agreement in September "upped the ante", which he said is having an effect on how people in Northern Ireland perceive the Good Friday Agreement.
"People who don't understand all the details in the unionist community or the nationalist community are now believing that this is all something to do with a constitutional position of Northern Ireland," he said.
"The constitutional position of Northern Ireland is set down in the Good Friday Agreement, and that was acknowledged in the withdrawal agreement," he said.
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In a strongly worded statement last night, the European Commission said the UK was in breach of the good faith provision of the Withdrawal Agreement, and was set to breach international law for a second time.
Last September, Northern Ireland Secretary Brandon Lewis told the House of Commons that the UK would breach international law by taking unilateral action on the protocol through the Internal Market Bill.
The UK suggests this is merely a technical and operational necessity because the grace period is running out at the end of this month, and because a plan for supermarkets to put in place a hi-tech traceability system will not be ready on time.
However, senior EU figures see this as a political exercise designed, once again, to bounce the EU into yielding to UK demands on how the protocol should be implemented.
Senior sources say the Commission has made genuine efforts to find workable solutions to the problems surrounding the protocol, but these have been swept aside by a British solo run.
Furthermore Commission Vice President Maroš Šefčovič, London's EU interlocutor, was not warned in advance.
Meanwhile, the SDLP leader Colum Eastwood has said that the Northern Ireland Protocol is the only alternative to a hard border on the island of Ireland and unionist leaders need to be "honest and stop pretending it will be scrapped".
Speaking on RTÉ's News at One, Mr Eastwood said that tensions are "very real" since the political parties went back into government in Northern Ireland and that "Brexit is at the core of all of these tensions".
Mr Eastwood said that there will be changes in supply chains that can be worked through, but not if the British government continue to do things unilaterally.
He said that the EU would have agreed to extending the grace period, so it was "reckless and performance art" by the British government to move to usurp the protocol and damage relations with the EU further.
Earlier, European Commissioner for Financial Stability, Financial Services and the Capital Markets Union said the Commission is disappointed with the UK's decision and that the UK is unilaterally negotiating itself a solution, rather than negotiating with the European Union.
Speaking on RTÉ's Morning Ireland, Mairead McGuinness said that while the UK painted this as a "simple solution to get over a problem" it is "very, very unusual" that one side would step outside negotiations and make a decision that impacts on a partner without discussion.
She said the EU is in the process of ratifying the trade and cooperation agreement and had hoped dates would be agreed today, and said she is not sure how the parliament will view this particular move by the UK.
There is no lack of will on the EU side, she said, and talks will have to continue but they will now continue "in a more difficult atmosphere than prior to this decision."
The commissioner said that the EU wants to address the problems that businesses have in Northern Ireland, adding that these problems were created by the type of Brexit that the British Prime Minister pursued.
However, she added, the response would be measured and the EU will be the adults in the room despite this "most unhelpful" development.
Ms McGuinness also said the EU is fully aware of the difficulties in Northern Ireland and will respond appropriately, adding that decisions will be made with the UK, "not without them".
However, she said, that they have to be clear when something happens that is not appropriate, and in their view breaches trust and an international agreement, they have to "call it out".
"It does open a question mark about global Britain if this is how global Britain would negotiate with other partners," the commissioner said.
She said their experience has not been an easy one "to put it mildly."