The EU has given Britain a two-month deadline to respond to legal action as it responded to Boris Johnson’s plan to tear up the Northern Ireland Protocol.
Maros Sefcovic, vice-president of the European Commission, said UK legislation to override parts of the protocol, which governs Northern Ireland’s post-Brexit trading arrangements, was illegal.
The commission confirmed that Brussels will resume legal proceedings against the UK, which it suspended in September, for breaching the EU withdrawal treaty agreed in 2020.
Mr Sefcovic said: “If the UK doesn’t reply within two months we may take them to the court of justice.”
Brussels is also launching two new legal infringement actions against the UK – but in addition offering what it called an “oven ready” solution to the impasse.
Mr Sefcovic said: “Let there be no doubt: there is no legal nor political justification whatsoever for unilaterally changing an international agreement.
“This is illegal. The UK bill is extremely damaging to mutual trust and respect between the EU and the UK.
“It has created deep uncertainty and casts a shadow over our international cooperation.”
The Northern Ireland Protocol was designed to avoid the return of a hard border with the Republic of Ireland as a result of Brexit.
But it has in effect created a border in the Irish Sea between Great Britain and Northern Ireland, meaning goods exported from Britain are subject to customs checks.
The UK government says that had created a headache for businesses and the power-sharing arrangements set up as a result of the Good Friday Agreement in 1998.
The unionist DUP has refused to rejoin the power-sharing executive until the problems are resolved.
Britain has claimed that attempts to renegotiate the deal with Europe have hit a dead end.
It decided to push ahead with legislation to change the protocol, which was published earlier this week, despite warnings it would be illegal under international law and could provoke a trade war with Europe.
The government has proposed scrapping some checks on goods arriving in Northern Ireland from the rest of the UK and challenged the role of the European Court of Justice in overseeing the implementation of parts of the protocol.
Europe’s legal ultimatum relates to action it originally launched a year ago over the UK’s unilateral decision to extend so-called grace periods – holding off the full implementation of border checks required under the protocol.
That action had been suspended in the light of talks between London and Brussels.
The two new legal proceedings – which do not specifically relate to the protocol legislation set out in Westminster – accuse the UK of failing to ensure adequate staff and infrastructure to carry out checks in Northern Ireland and not providing the EU with sufficient trade data.
Mr Sefcovic also illustrated Europe’s proposals for easing customs formalities by holding up, during a news conference, a three-page sample certificate that would accompany a lorry carrying multiple goods under the plans.
“Not 300, not 30, three. This is how simple it is and what we can do if we work well together,” he said.
“What we are putting on the table is oven-ready.”