The European Commission launched new legal action against the United Kingdom on Wednesday after London published plans to override some rules of the post-Brexit trade deal on Northern Irish trade, and resumed another challenge it had previously paused.
The proceedings could result in the European Court of Justice (ECJ) imposing fines, although these would likely be more than a year away.
London has proposed scrapping some checks on goods from the rest of the United Kingdom arriving in the British province and challenged the role of the ECJ to decide on parts of the post-Brexit arrangement agreed by the European Union and Britain.
European Commission Vice President Maros Sefcovic, who oversees EU relations with former EU member Britain, said there was no justification for unilaterally changing an international agreement
"Let's call a spade a spade. This is illegal," he told a news conference, adding it cast a shadow on relations at a time when international cooperation was even more important, a reference to the alliance against Russia's invasion of Ukraine.
The three legal proceedings do not relate to Britain's new plans, but to the EU belief that Britain has failed to implement the protocol that governs Northern Irish trading.
The two new suits charge Britain with failing to ensure adequate staff and infrastructure to carry out checks in Northern Ireland and not providing the EU with sufficient trade data.
The other, paused a year ago to improve the atmosphere around talks, relates to the movement of agri-food products. Sefcovic said that the EU might take the case to the ECJ if Britain failed to address the EU's charges within two months.
Sefcovic said Brussels still wanted to resume talks with Britain to resolve difficulties in shipping British products to Northern Ireland.
"We decided that our response should be measured, should be proportionate. And we are offering not only legal action here today but we've been fleshing out what concretely we could do," he said.
The British province is in the EU single market for goods, meaning imports from the rest of the United Kingdom are subject to customs declarations and sometimes require checks on their arrival. The arrangement was set to avoid reinstating border controls between Northern Ireland and EU member Ireland, which were dropped after the 1998 Good Friday peace agreement.
The arrangement has inflamed pro-British unionist parties by effectively creating a border in the Irish Sea.
The commission made a series of proposals last October to ease customs formalities and cut checks and on Wednesday published a sample three-page certificate that would accompany a truck carrying multiple goods on Wednesday.
"Not 300, not 30, three. This is how simple it is and what we can do if we work well together," Sefcovic said, holding up the certificate.
"What we are putting on the table is oven-ready," he said, in reference to British Prime Minister Boris Johnson's 2019 pre-election promise about passing the Brexit divorce deal, while adding this was not a "take-it-or-leave-it" offer.
