The European Commission said it launched the infringement proceeding because the UK has failed to implement the agreement “despite repeated calls” to do so.
The UK government argues the agreement needs to be “fixed” to avoid “burdensome customs processes, inflexible regulation, tax and spend discrepancies and democratic governance issues.”
The EU said renegotiating the protocol was “unrealistic” and that changing it unilaterally would be considered a breach of an international agreement, which could result in fines.
Speaking to reporters on Wednesday, Maroš Šefčovič, the European Commission Vice-President, 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,” he added.
The agreement was put in place to safeguard the Good Friday Agreement, which helped end years of deadly sectarian violence and which mandates that there should be no hard border between the Republic of Ireland, which is part of the EU, and Northern Ireland, which has left the EU alongside the rest of the UK.
To avoid a hard border, the UK has agreed to keep Northern Ireland within the EU regulatory scheme. That solution, however, created another headache: because the rest of the UK does not fall under EU rules, goods leaving Northern Ireland for the rest of the UK would have to be checked.
Despite agreeing to this solution, the British government now says this solution is unfair.
“[The change of the agreement] will end the untenable situation where people in Northern Ireland are treated differently to the rest of the United Kingdom, protect the supremacy of our courts and our territorial integrity, “UK Foreign Secretary Liz Truss said in a statement earlier this week.
Speaking to reporters on Wednesday, Šefčovič, said that aside from launching the legal procedure, the EU was also putting forward some “additional details” on the possible solutions it has suggested earlier.
However, he also acknowledged the spat could escalate further if the UK goes ahead with the changes, even sparking a trade war. “But we are not there yet and we want to resolve this issue as the two partners should, through negotiations, looking for the common ground and delivering for the people of Northern Ireland,” he said.
The EU said in a statement it would take the UK to the European Court of Justice if the British government does not respond within two months.