The European Commission has formally taken legal action against the UK for what it describes as a "deliberate" breach of international law over its unilateral action on the Northern Ireland Protocol.

In a strongly worded letter to the UK's lead minister on Britain's relationship with the EU, David Frost, the Commission vice-president Maroš Šefčovič accused London of unilaterally departing from the rules of the protocol, and of not implementing the measures agreed by both sides.

In the letter Mr Šefčovic said: "The UK has resorted to this unilateral action without any discussion or consultation with the EU side in the bodies established by the [Withdrawal] Agreement.

"It has therefore acted in breach of the mutual trust and spirit of cooperation that we managed to rebuild in the last months of 2020, after the uncertainty created by the UK Internal Market Bill.

"The recent measures once again set the UK on a path of a deliberate breach of its international law obligations and the duty of good faith that should prevail in the application of international agreements pursuant to Article 26 of the Vienna Convention on the Law of Treaties."

In a statement the Commission said the moves announced by the Northern Ireland Secretary on 3 March were "the second time in the space of six months that the UK government is set to breach international law".

Mr Šefčovic's "political" letter to his UK opposite number, which accuses the UK of being in breach of the good faith provisions of the Withdrawal Agreement, is one of two legal moves by the European Commission, the second being a letter of formal notice which triggers an "infringement" procedure against the UK.

That letter is more specifically a charge that the UK is in breach of EU law by not complying with the checks and controls contained within the protocol.

Minister for Foreign Affairs, Simon Coveney, said on Twitter: "Legal Action is not a welcome development, but approach of UK Government has given EU no alternative.

"Unilaterally changing how Protocol is implemented is breach of Agreement. We need to get back to UK/EU cooperation, working with business in NI and focused on solving problems together."

A UK Government spokesperson said: "We have received the letters from the European Commission and will respond in due course.

"We have been clear that the measures we have taken are temporary, operational steps intended to minimise disruption in Northern Ireland and protect the everyday lives of the people living there.

"They are lawful and part of a progressive and good faith implementation of the Northern Ireland Protocol.

"Low key operational measures like these are well precedented and common in the early days of major international treaties.

"In some areas, the EU also seems to need time to implement the detail of our agreements. This is a normal process when implementing new treaties and not something that should warrant legal action.

"These aspects of the Northern Ireland Protocol have only been in force for 70 days and we saw the challenges faced by supermarkets and others in the early weeks of January as a result of the Joint Committee agreement only being reached in December," the statement continued.

"That's why it is right to provide a proper further period for them to plan ahead, particularly in the current circumstances of a global pandemic.

"All sides need to keep in mind the fact that the Protocol depends on cross-community consent and confidence if it is to work and deliver our common objective of protecting the Belfast (Good Friday) Agreement in all its dimensions, North-South and East-West.

"We look to continue discussing the issues within the Joint Committee framework in a constructive fashion."

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An EU official said the reality of checks and controls on some goods moving from Britain to Northern Ireland was "crystal clear" to the UK when it negotiated the protocol in September and October 2019.

"The UK government and the then chief negotiator [David Frost], who is now minister of state, made a choice in Sept 2019," said the official. 

"Then there was a renegotiation of the Withdrawal Agreement based on the premise that there would be checks on some goods moving east west, from Great Britain to Northern Ireland.

"That was really crystal clear to all sides and it was the working assumption of the talks at the time," said the official.

EU sources said the facts and the UK's actions had shown that it was not implementing the protocol in full, as it had promised.

The UK had chosen not to extend the 11-month transition period, which was there "specifically to enable everybody to understand the consequences of the new arrangements and the new reality".

An EU official said the UK had been clear that the grace periods, agreed with the EU in December, were "sufficient" to allow all operators to be ready to apply the protocol by 1 April.

"The EU looked at these requests, understood and discussed the conditions that would accompany those requests, showed understanding and flexibility, and accepted that in practice more time was needed, and so these grace periods could in fact apply," said the official.

The official said that in return for the grace periods the UK would provide a road map, or "a clear understanding of when and how we would get a pathway towards a full implementation of the protocol".

The official added: "This is something we have not received. In other words we have not received the UK roadmap explaining what it would do in practice in order for the protocol to be applied in full".

On 3 March, Secretary of State for Northern Ireland Brandon Lewis said the UK would extend one of the mutually agreed grace periods from 1 April until 1 October.

This would mean suppliers could continue sending products of animal origin, composite products, food and feed of non-animal origin and plants and plant products from GB to Northern Ireland without official EU certification.

In his letter to David Frost, Maroš Šefčovič said the UK had taken this decision "without any discussion or consultation with the EU side".

Furthermore, the UK had also action on the movement of pets and express parcels from GB-NI which amounted to the "unilateral disapplication" of the protocol.

These latter moves had not been the subject of discussion between the EU and UK, the letter states.

The European Commission has given London one month to respond to the letter of formal notice.

If there is no response, the Commission could issue what is called a "reasoned opinion", which in turn could be a precursor to a case being taken to the European Court of Justice, which could impose fines.

In his letter, Maroš Šefčovič demanded "swift remedial action" by the UK in order to comply with the protocol, asking Mr Frost that the UK "rectify and refrain from putting into practice" the unilateral moves announced on 3 March.

In its statement, the Commission said that if the UK failed to enter into consultations in the Joint Committee "in good faith, with the aim of reaching a mutually agreed solution by the end of this month," the EU would trigger the dispute settlement procedure contained within the Withdrawal Agreement.

If the UK failed to comply with arbitration on the matter, it could result in fines, or in the EU imposing tariffs on UK goods.

Taoiseach Micheál Martin has said that while Brexit has "created some challenges", he believes they can be overcome if both the EU and UK "use the mechanisms" that they jointly negotiated.

Speaking on US network CNBC, he said it was worth remembering that the Brexit deal had resulted in a "no-tariff no-quota" trading arrangement and that was something to be welcomed.

Mr Martin said that while the UK was entitled to pursue any trade deal with the US, Ireland wants London to work with the European Union to "safeguard and underpin" the Withdrawal Agreement and the Northern Ireland protocol.

Additional reporting Paul Cunningham