The European Union on Monday threatened legal action after Britain’s government proposed new legislation that would unilaterally change post-Brexit trade rules for Northern Ireland, despite opposition from some U.K. lawmakers and EU officials who say the move violates international law.

The proposed bill seeks to remove customs checks on some goods entering Northern Ireland from the rest of the U.K. That will override parts of the trade treaty that Prime Minister Boris Johnson signed with the European Union less than two years ago.

Britain's government maintains it is acting within international law, but the European Commission said it could take legal action against the U.K.

European Commission Vice President Maros Sefcovic said the EU’s executive arm will consider launching new infringement procedures to "protect the EU single market from the risks that the violation of the protocol creates for EU businesses and for the health and safety of EU citizens."

In Ireland, Foreign Affairs Minister Simon Coveney said the bill "marks a particularly low point in the U.K.'s approach to Brexit." The Irish prime minister, Micheal Martin, said it was "very regrettable for a country like the U.K. to renege on an international treaty."

The legislation is effectively a breach of international law and nobody but London thinks otherwise, Convey said.

"it’s effectively a breach of international law should this legislation become law. I haven't met anybody outside of the British government that thinks that it isn't a breach of international law when you deliberately disapply an international treaty," Coveney told national broadcaster RTE.

Coveney added he did not believe there was any good reason for the move as the European Union is looking to negotiate with Britain to find a compromise that would solve the issues some businesses are having due to the rules.

Brushing aside criticism, Johnson told reporters that the proposed change is "relatively simple to do."

"Frankly, it’s a relatively trivial set of adjustments in the grand scheme of things," he told LBC Radio.

He argued that his government's "higher and prior legal commitment" is to the 1998 Good Friday agreement that brought peace and stability to Northern Ireland.

Arrangements for Northern Ireland – the only part of the U.K. that shares a land border with an EU nation – have proved the thorniest issue in Britain’s divorce from the bloc, which became final at the end of 2020. At the center of the dispute is the Northern Ireland Protocol, which regulates trade ties between Northern Ireland, which is part of the U.K., and the Republic of Ireland, part of the EU, after Brexit.

Britain and the EU agreed in their Brexit deal that the Irish land border would be kept free of customs posts and other checks because an open border is a key pillar of the peace process that ended decades of violence in Northern Ireland.

Instead, to protect the EU’s single market, there are checks on some goods, such as meat and eggs, entering Northern Ireland from the rest of the U.K.

But the arrangement has proved politically damaging for Johnson because it treats Northern Ireland differently from the rest of the United Kingdom, potentially weakening the province’s historical links with Britain. Northern Ireland’s Democratic Unionist Party has refused to return to the region’s power-sharing government until the protocol is scrapped or substantially changed to address those concerns.

The bill to override that arrangement is expected to face opposition in Parliament, including from members of Johnson's Conservatives. Critics say unilaterally changing the protocol would be illegal and would damage Britain’s standing with other countries because it's part of a treaty considered binding under international law.

In Brussels, Sefcovic said the protocol was the "one and only solution we could jointly find to protect the hard-earned gains of the peace process in Northern Ireland." He added that the EU will not renegotiate the protocol.

The U.S. on Monday urged Britain and the EU to return to talks to resolve differences over the implementation of the Northern Ireland Protocol.

"U.S. priority remains protecting the gains of the Belfast/Good Friday Agreement and preserving peace, stability, and prosperity for the people of Northern Ireland," a White House spokesperson said.