Britain plunged Brexit trade talks into crisis on Wednesday by publishing a bill that explicitly acknowledges the government could break international law by ignoring some parts of the divorce treaty it signed with the European Union.
Brushing aside warnings from Brussels that breaching the treaty would prevent any trade deal being struck, London said in the proposed legislation that it would ignore parts of the Withdrawal Agreement, which was only signed in January.
The Internal Markets Bill says that certain provisions are "to have effect notwithstanding inconsistency or incompatibility with international or other domestic law".
The government has said international law would be broken "in a very specific and limited way", but the EU has made its anger plain.
"Very concerned about announcements from the British government on its intentions to breach the Withdrawal Agreement," Ursula von der Leyen, president of the European Commission, the EU executive, said on Twitter.
"This would break international law and undermines trust. Pacta sunt servanda = the foundation of prosperous future relations," she said. The Latin phrase, meaning "agreements must be kept", is a basic principle of international law.
British Prime Minister Boris Johnson told parliament the bill was "a legal safety net to protect our country against extreme or irrational interpretations" of the Northern Ireland protocol of the Withdrawal Agreement that could threaten peace in the British province.
The bill, if approved, would give ministers the power to ignore parts of the protocol by modifying the form of export declarations and other exit procedures.
It will be debated in both chambers of parliament and require their approval before becoming law.
Britain quit the EU in January but has remained part of its single market, largely free of trade barriers, under a status quo agreement that expires in December. It has been negotiating a trade deal to take effect from Jan. 1, but says it is willing to walk away if it cannot agree favourable terms.
The British pound, which tends to rise with the perceived likelihood of a negotiated trade deal with the EU, was down around 0.4% in mid-afternoon at $1.2939.
Irish Prime Minister Micheal Martin said he would speak to Johnson to express "very strong concerns" about the plans while his deputy Leo Varadkar called it a "kamikaze" threat that had backfired.
Asked how he could expect Britons to obey the law if his government was willing to undermine it, Johnson said: "We expect everybody in this country to obey the law."
Senior members of Johnson's Conservative Party have already voiced anger that Britain might consider such a move.
Scotland and Wales said the bill would weaken the fabric of the United Kingdom itself by stealing powers from Wales, Scotland and Northern Ireland.
"The prime minister and his friends ... are creating a rogue state, one where the rule of law does not apply," Ian Blackford, leader of the pro-independence Scottish National Party, told the Westminster parliament.