The European Union has triggered legal action against the UK in a major escalation of tensions between the two sides.
Just under three months after Brexit was formally completed, EU Brexit chief Maros Sefcovic wrote to the Government to formally begin legal proceedings for the second time in six months.
The move comes after No10 announced plans to unilaterally delay the introduction of customs checks on supermarket and other agricultural goods being shipped from mainland Britain to Northern Ireland.
Checks will be needed because Northern Ireland is in the UK customs territory but the EU single market, and has a free-flowing border with the EU-member Republic of Ireland.
Grace periods allowing goods to freely cross the Irish Sea had been due to run out as soon as April 1.
The EU's decision could ultimately lead to financial penalties or trade tariffs being imposed against the UK.
Or it could see the Government defend itself in the European Court of Justice over the alleged breach.
Ministers argue that the temporary measures are designed to give people and businesses extra time to adapt to life under the Brexit divorce deal’s protocol to avoid a hard border.
But the row is likely to worsen the already fraught relationship between the two sides that has led to spats over the export of Covid vaccines.
The UK is also refusing to grant full rights to Brussels' ambassador in London.
Commission vice president Maros Sefcovic accused the Government of a breach of trust.
He said the UK's actions had "once again set the UK on a path of a deliberate breach of its international law obligations".
But the UK government hit back at Brussels today, insisting the measures were "temporary, operational steps to minimise disruption in Northern Ireland."
Prime Minister Boris Johnson defended the extensions as "very sensible".
A "letter of formal notice" was sent to Westminster on Monday over the latest alleged breach, marking the beginning of the formal infringement process.
It requests that the UK carries out "swift remedial action to restore compliance with the terms of the protocol", offering a potential olive branch to prevent a legal battle.
After a call on Monday morning, Mr Sefcovic sent a separate "political" letter to Lord Frost, the Cabinet Office minister who negotiated the Brexit deal.
In a briefing in Brussels, an EU official told reporters that the UK had already committed the second breach causing an "enormous problem" with "real-life issues".
"Let's be clear about what the United Kingdom is doing with these unilateral announcements," the official said.
"They are actively telling stakeholders not to apply an international agreement so yes we believe that is a violation."
A UK government spokesperson said: "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.
"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."
