Britain will propose overriding the rules that govern post-Brexit trade with Northern Ireland in a move that will inflame tensions with the European Union that have simmered since the protocol came into force in 2021.

When Britain left the EU, Prime Minister Boris Johnson agreed to a deal that effectively kept Northern Ireland in the EU single market and customs union to preserve the open border with Ireland specified in the Good Friday peace agreement.

It imposes customs checks between the province and the rest of the United Kingdom, which pro-British communities in Northern Ireland say erodes their place within the UK.

Mr Johnson has said the protocol’s implementation has damaged trade within the United Kingdom and has threatened political stability in Northern Ireland.

Foreign Secretary Liz Truss will present the legislation to parliament on Monday.

Brussels believes any unilateral change could breach international law. It could respond by launching legal action and by imposing countermeasures, such as tariffs.

Northern Ireland Secretary Brandon Lewis insisted on Sunday (local time) the legislation complied with the law, but declined to say how the protocol would be changed.

The bill is expected to propose a “green channel” for goods moving from Britain to Northern Ireland, as well as scrapping rules that prevent the province benefiting from tax assistance and ending the role of the European Court of Justice as sole arbiter, according to reports.

The plan will be a test of Mr Johnson’s authority after he was almost removed from office last week when four in 10 of his MPs opposed him in a confidence vote.

However, it will be seen by the EU as an inflammatory move that violates an international treaty.

European Commission vice-president Maros Sefcovic said in May that Brussels would respond with all measures at its disposal.

Ireland’s Sinn Fein, the nationalist party that won a historic victory in an election in Northern Ireland last month, said Britain would “undoubtedly” break the law by imposing unilateral changes to the protocol.