The Taoiseach has described the decision to resume legal action against the UK over its move on the Northern Ireland Protocol as a step up in the response from the EU.
However, Micheál Martin said the European Union is still available and wants to bring a resolution to issues surrounding the protocol.
Mr Martin again called on the British government to engage in substantive negotiations.
"This is the only logical and rational way forward," he said.
The EU's announcement that it is to resume legal action against the UK follows the publication of a bill that would overturn the Northern Ireland Protocol.
European Commission vice-president Maroš Šefčovič told reporters in Brussels the UK government had set out to "unilaterally break international law".
The plan would mean "breaking an agreement that protects peace and stability in Northern Ireland, an agreement reached together only three years ago" by Boris Johnson's government and the EU.
Mr Šefčovič said: "Let there be no doubt: there is no legal nor political justification whatsoever for unilaterally changing an international agreement.
"Opening the door to unilaterally changing an international agreement is a breach of international law as well.
"So let's call a spade a spade: this is illegal."
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Reacting to the EU announcement, Micheál Martin said: "I have been in regular contact with Maroš Šefčovič, with President [Ursula] von der Leyen and they are anxious to engage on the detail and on the technical issues around the protocol."
The Taoiseach said the EU's legal action against the UK is a "step up in a response" from the EU. However, he said that the EU is still available and wants to bring a resolution to issues arising from the protocol "through substantive negotiations" | https://t.co/IdpAt3Nipa pic.twitter.com/9yig4ma5du— RTÉ News (@rtenews) June 15, 2022
A senior EU official has warned that the UK could be brought before the European Court of Justice within two months over unilateral action that London took last year.
The commission has also announced fresh legal action over the alleged failure of the UK to build and staff border control posts at Northern Ireland ports and an alleged failure by the UK to provide real time data on the movement of goods between Great Britain and Northern Ireland.
The European Commission has also revealed what it calls large scale attempted smuggling of goods into the single market via Northern Ireland.
A senior EU official said that last year there were seizures at northern ports of weapons, counterfeit smartphones, heroin, cocaine, tobacco and counterfeit medicines, which were potentially destined for the EU single market.
The official said the UK bill would give British ministers the powers to disapply all but three elements of the protocol.
European Commission vice-president Maroš Šefčovič told reporters in Brussels the UK government had set out to "unilaterally break international law" the publication of the bill that would overturn the Northern Ireland Protocol | https://t.co/iGGqGpzpcT pic.twitter.com/OOXh6gzsSw— RTÉ News (@rtenews) June 15, 2022
He said those three elements, which include the Common Travel Areas, North-South cooperation and citizens' rights, are all areas that were active before Brexit and before the protocol.
UK 'disappointed' over EU move
Britain is disappointed with the EU's move in relation to legal proceedings against it and will review the documents carefully before responding, the Prime Minister's spokesman said.
"We will review these documents carefully and respond formally in due course. However, we are disappointed that the EU has taken this legal action today," the spokesman told reporters.
"The EU's proposed approach, which doesn't differ from what they've said previously, would increase burdens on businesses and citizens and take us backwards from where we are currently," he said, referring to EU proposals to ease post-Brexit trade problems with Northern Ireland.
The European Commission has also set out further detail on its own proposals, published last October, to reduce the burden of the protocol.
A senior official said that, under the EU's proposals, an entire truck of goods moving from a Sainsburys depot in the UK to a branch in Northern Ireland would only need three pages of documentation.
He said this offer was a "fleshing out" of the EU's position and unilateral action "was not at all necessary".
The official said the UK bill was a clear breach of international law, even if it did not enact the legislation.
"Any action by a party to an international agreement to disapply an international agreement unilaterally is a breach of international law, and creating the possibility to disapply unilaterally that agreement is also a breach of international law," the official said.
The official said the UK bill essentially meant that the UK would be deciding what could and could not enter the EU's single market.
The official said: "It is legally and politically inconceivable that the UK sets these conditions unilaterally. It is simply not acceptable to the EU."
The official said the Northern Ireland Protocol meant that EU single market rules for goods applied, and the only court that could interpret those rules was the European Court of Justice.
The court was there for the benefit of traders both in the EU and in Northern Ireland, he said.
He said it would be unacceptable for the court to be removed from the equation, and the court itself would regard that as illegal.
Despite the deepening standoff, a second senior EU official said the EU would resume talks with the UK "tomorrow", if the UK returned to the proposals the European Commission presented last October.
Officials have said the UK’s ideas for a green lane for goods not moving into the single market were not dissimilar to the EU’s idea of an express lane.
The difference was that the EU required some checks and data in order to make a risk assessment of what was entering Northern Ireland, and to be sure that the goods were in fact not crossing the border into the south.
This is NOT where we want to be.— Simon Coveney (@simoncoveney) June 15, 2022
But this is the result of a deliberate UK Government strategy of provocation over partnership.
Reckless UK decisions this week have forced #EU into responding to a threatened breach of Int. Law with serious consequences. https://t.co/nUwBnDZge6
The British Ambassador to Ireland, Paul Johnston, has said that the UK government is tabling a bill to amend the protocol because it "isn't working as well as we would want".
"We'd like to make changes, ideally through negotiation with the European Union... We haven't got there yet, so we're introducing this legislation," he said.
Speaking on RTÉ's Morning Ireland, Mr Johnston said the British government was proposing its legislation "still in the hope that as we take the legislation forward, there will also be scope to take negotiation forward".
In relation to the European Commission's proposals for improvements to the protocol, published in October, Mr Johnston said these were an "important effort", but that they had proposed mitigating parts of the protocol that had not yet been implemented which, the ambassador said, "no-one is talking about any more".
"It's really important that each side continues to clarify misunderstandings, address points that are in dispute, and also discuss the way forward," he added.
Additional reporting PA