Brussels said it saw the UK’s decision to unilaterally extend post-Brexit grace periods on trade in Northern Ireland as a violation of international law.
“It is the second time in the space of six months that the UK government is set to breach international law,” the European Commission said.
If the UK does not respond or come back to the negotiating table on the measures, it could face the prospect of penalty fines or ultimately the imposition of tariffs on exports to the EU.
The prime minister has argued that the grace period extensions, widely attributed to his newly promoted Brexit minister Lord Frost, are “very sensible”.
The government has denied breaching the terms of the Northern Ireland protocol, which is part of the withdrawal agreement (WA) negotiated by Johnson and Frost.
The protocol was designed by the UK and EU to avoid a hard border on the island of Ireland, keeping Northern Ireland in the European single market for goods – but crucially creating the need for checks and import regulations on products arriving from Great Britain over the Irish Sea.
The first of the grace periods suspending certain checks, including on supermarket supplies and parcel deliveries, had been due to expire at the end of March.
But the UK pledged to extend them until October in a move widely welcomed by businesses in Belfast, but which was not agreed with the EU.
Maros Sefcovic, the EU’s co-chair of the joint committee that oversees the WA, said: “The EU and the UK agreed the protocol together.
“We are also bound to implement it together.
“Unilateral decisions and international law violations by the UK defeat its very purpose and undermine trust between us.
“The UK must properly implement it if we are to achieve our objectives.
“That is why we are launching legal action today.
“I do hope that through the collaborative, pragmatic and constructive spirit that has prevailed in our work so far on implementing the withdrawal agreement, we can solve these issues in the joint committee without recourse to further legal means.”
The EU has sent the UK a formal letter of notice of legal action.
Sefcovic has also sent a “political letter” to Frost, urging the UK to “refrain” from taking unilateral action and to enter negotiations in the joint committee “with the aim of reaching a mutually agreed solution by the end of this month.”
The UK has one month to reply to the letter of formal notice.
If the UK does not respond or the response is unsatisfactory, the EU can begin proceedings which could end in the European Court of Justice imposing a lump sum or penalty payment, the Commission said.
If the UK fails to enter consultations with Brussels through the joint committee “in good faith”, the EU can start separate dispute resolution proceedings under the terms of the WA.
If no solution is found through that measure, the EU can refer the dispute for arbitration, which could result in the UK suffering financial sanctions.
If the UK refuses to pay or continues to break the rules, the EU can suspend its obligations under both the WA and wider trade and cooperation agreement signed at the end of last year, and impose tariffs on British exports to Europe.
Downing Street was preparing to issue a full response after it received the letter.
The PM’s official spokesperson said that the measures taken are “temporary operational steps intended to minimise disruption in Northern Ireland”.
They said the UK remains committed to the protocol, “but there are issues that have emerged since the implementation of the protocol” and that work is being addressed through the joint committee process bringing together both sides.
It is the second time tensions have reached a head over the implementation of the WA.
The government admitted last year that its UK internal market bill, required for the UK to depart the EU’s single market and customs union, could allow ministers to breach international law.
But the offending sections of the legislation were removed after both sides reached an agreement.