The United Kingdom and the European Union were once again at loggerheads over Brexit on Wednesday, after the U.K. government ramped up threats to scrap parts of its trade treaty with the bloc, saying the rules are blocking the formation of a new government in Northern Ireland.

Foreign Secretary Liz Truss said the government would "not shy away from taking action" if it can't reach agreement with the bloc.

The EU warned that renegotiating the legally binding agreement "is not an option." Any move by Britain to unilaterally rewrite the rules would bring legal action from the bloc that could escalate into a trade war.

Arrangements for Northern Ireland – the only part of the U.K. that shares a border with an EU nation – have been the thorniest subject of contention in the U.K.’s divorce from the 27-nation bloc, which became final at the end of 2020.

A deal was agreed to keep the Irish border 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, there are checks on some goods entering Northern Ireland from the rest of the U.K.

The arrangement is opposed by many of Northern Ireland’s British unionists, who say the new checks have created a barrier with the rest of the U.K. that undermines their British identity. The Democratic Unionist Party, which came second in last week’s Northern Ireland Assembly election, is refusing to help form a government until the arrangements, known as the Northern Ireland Protocol, are substantially changed or scrapped.

Under Northern Ireland’s power-sharing rules, a government can’t be formed without the support of both the main unionist and nationalist parties. Sinn Fein won the most seats last week, the first time a party that seeks to unite Northern Ireland with the republic has topped the voting.

U.K.-EU talks on resolving differences over trade rules have reached an impasse. Britain’s Conservative government has accused the bloc of being needlessly "purist" in its approach to the rules, while the EU says Britain is failing to honor a legally binding deal that Prime Minister Boris Johnson agreed to.

EU chief Brexit official Maros Sefcovic said Tuesday that the bloc had "worked tirelessly to propose creative and durable solutions, showing flexibility on how the Protocol should be implemented."

He warned that "the Protocol, as a cornerstone of the Withdrawal Agreement, is an international agreement. Its renegotiation is not an option. The European Union is united in this position."

Truss, who is in charge of negotiations with the bloc, said in a statement that EU proposals "fail to properly address the real issues affecting Northern Ireland and in some cases would take us backward."

"Our preference has always been for a negotiated solution but we will not shy away from taking action to stabilize the situation in Northern Ireland if solutions cannot be found," she said.