The EU has announced fresh legal action against the UK after Boris Johnson unveiled plans to rip up his own Brexit deal.
European Commission Vice President Maros Sefcovic branded the move "illegal" and said Brussels had been forced to act after the UK Government published draft legislation allowing it to scrap parts of the agreement.
Tensions between the UK and the EU have risen over a key part of the PM's Brexit deal relating to Northern Ireland, which has angered unionist politicians and caused some disruption to trade.
Foreign Secretary Liz Truss published a new bill on Monday allowing the UK to change parts of the Northern Ireland Protocol - which Boris Johnson negotiated - without asking permission from Brussels.
Mr Sefcovic told a press conference: "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."
Boris Johnson's spokesman hit back: "We will review these documents carefully and respond formally in due course, however, we are disappointed that the EU has taken this legal action today.
"The EU's proposed approach, which doesn't differ from what they've said previously, would increase burdens on businesses and citizens and take us backwards from where we are currently," he said, referring to EU proposals to ease post-Brexit trade problems with Northern Ireland."
He added: "The UK Government's decision has left us with no choice but to act."
Mr Sefcovic said the legislation was "extremely damaging to mutual trust and respect between the EU and the UK".
He said: "It has created deep uncertainty and casts a shadow over our overall co-operation, all at a time when respect for international agreements has never been more important.
"That is why the Commission has today decided to take legal action against the UK for not complying with significant parts of the protocol on Ireland/Northern Ireland."
The EU is reviving legal action relating to the UK's decision to delay imposing checks on certain goods arriving in Northern Ireland from Great Britain - which was put on hold in 2021.
If the UK doesn't respond in the next two months, the bloc will refer the complaint to European Court of Justice.
It has also launched new proceedings over claims the UK has failed to share trade data and set up border inspection posts.
Mr Sefcovic said the EU remains open to dialogue but warned it was "inconceivable" for the UK to make decisions without consulting Brussels.
Ministers have been threatening to rip up the Northern Ireland Protocol for months but the issue has become more pressing after Sinn Fein's recent election success at Stormont.
The nationalists became the largest party in the Northern Ireland Assembly last month but they have been unable to resume powersharing as the unionist DUP party loathe the protocol.
A UK Government spokesperson said: “It is disappointing that the EU has chosen to relaunch legal proceedings relating to the grace periods currently in place, which are vital to stop the problems caused by the Protocol from getting worse.
"The UK’s preference remains for a negotiated solution but the proposals set out by the EU today are the same proposals we have been discussing for months and would not solve the problems – in many cases they take us backwards from current arrangements.
“The Protocol is undermining the Belfast (Good Friday) Agreement – disrupting trade and leading to people in Northern Ireland being treated differently to the rest of the UK.
"The EU continues to insist it is unwilling to change the Protocol itself, so we are obliged to act ourselves to change the parts of the Protocol that are causing problems.
"The legislation we introduced this week delivers practical, reasonable solutions which also protect the EU single market and support North-South trade.”
