LONDON (AP) — Britain’s government is expected to introduce legislation Monday that would unilaterally change post-Brexit trade rules for Northern Ireland amid opposition from lawmakers who believe the move violates international law.

The legislation would let the government bypass the so-called Northern Ireland Protocol, which requires the inspection of some goods shipped there from other parts of the United Kingdom. The protocol, designed to preserve free trade on the island of Ireland, is part of the broader trade deal that Prime Minister Boris Johnson negotiated with the European Union when Britain left the 27-nation bloc.

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 historic links with Britain. Northern Ireland’s Democratic Unionist Party has refused to return to the region’s power-sharing government until the protocol is amended to address those concerns.

The opposition Labour Party, and even some members of Johnson’s Conservatives, say unilaterally changing the protocol would be illegal and would damage Britain’s standing with other countries because its part of a treaty considered binding under international law.

“Breaking international law to rip up the Prime Minister’s own treaty is damaging to everything the U.K. and Conservatives stand for,” opponents of the bill said in a note being circulated among Conservative lawmakers, according to the Financial Times.

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.

The 1998 Good Friday agreement that ended decades of violence in Northern Ireland was underpinned by the fact that both the U.K. and Ireland were EU members, allowing goods and people to flow freely across the border.

While both sides are committed to keeping the border open, other competing interests have made that difficult to achieve.

The EU, focused on protecting its internal market, wants to ensure that all goods flowing into the bloc meet its standards. Britain is trying to assert its newly won independence from the EU while preserving the union of England, Northern Ireland, Scotland and Wales.

After a conversation with British Foreign Secretary Liz Truss, European Commission Vice President Maros Sefcovic said “unilateral action is damaging to mutual trust and a formula for uncertainty.”

The EU’s executive arm is expected to give a more detailed response once the U.K.’s plans have been released.

Ireland’s foreign affairs minister, Simon Coveney, also spoke with Truss on Monday and accused the U.K. government of deliberately ratcheting up “tension with an EU seeking compromise.”

Associated Press reporter Samuel Petrequin in Brussels contributed to this story.

Follow AP's coverage of Brexit at https://apnews.com/hub/brexit.