At the heart of the disputes is the international treaty the British prime minister negotiated in 2019 to secure the UK's departure from the European Union, breaking a long impasse.

The government says it will have "no choice but to act" unless the Northern Ireland Protocol is reformed, amid reports that it is about to introduce legislation to unravel it. It insists it wants to reform the agreement, not scrap it.

The Democratic Unionist Party (DUP), long opposed to the protocol, came second in recent elections and is refusing to form a government, or even allow the Northern Ireland Assembly to sit, while the treaty's provisions remain in force.

Under power-sharing rules set up as part of Northern Ireland’s peace process, a government can’t be formed without the cooperation of both nationalist and unionist parties.

Voters in Northern Ireland elected a new Assembly this month, in an election that saw Irish nationalist party Sinn Fein win the most seats. It was the first time a party that seeks union with the Republic of Ireland has won an election in the bastion of Protestant unionist power.

Many unionists are furious over post-Brexit checks on goods entering Northern Ireland from the rest of the UK, as required under the deal. The protocol keeps the north subject to many EU rules in order to preserve an open land border with the Republic of Ireland.

Johnson will urge political leaders in Belfast to get back to work and deal with “bread and butter” issues such as the soaring cost of living, his office said on Sunday.

It said he will also accuse the EU of refusing to give ground over post-Brexit border checks and warn that Britain will have a “necessity to act” unless the bloc changes its position.

The prime minister's office said that the trade agreement — which Johnson's government negotiated and signed — had “resulted in the unionist community feeling like its aspirations and identity are threatened.”

If the UK does act unilaterally to override the protocol — part of the legally-binding Brexit divorce deal — it leaves itself open to retaliation from the EU, including potentially trade sanctions. The 27-nation bloc is Britain’s biggest economic partner.

Ivan Rogers, a former British ambassador to the EU, said, “I think there’s a severe risk that we are heading into a trade war.”

Irish Foreign Minister Simon Coveney said Britain’s “saber-rattling and grandstanding” was undermining Northern Ireland peace “at a time when the world needs the Western world to be united, to be acting in concert to solve problems together.”

“The last thing the EU wants, the last thing that Ireland wants, is tension with the UK, particularly at the moment given what’s happening in Ukraine, Russian aggression, and the need to work together on an international stage,” he told Sky News.

Last week both sides published fresh statements setting out their stances, each one accusing the other of ignoring its own proposals to find solutions.

British Foreign Secretary Liz Truss said the protocol was "the greatest obstacle" to forming a Northern Ireland administration, was "causing unacceptable disruption to trade" and was leading to unfair treatment for the people of Northern Ireland.

But the European Commission's Brexit negotiator Maroš Šefčovič said the UK's threat of unilateral action was of "serious concern".

“This is despite a series of wide-ranging and impactful solutions proposed by the EU, based on our intensive engagement with all representatives in Northern Ireland. These proposals would substantially improve the way the Protocol is implemented," he wrote.

On Sunday British Business Secretary Kwasi Kwarteng accused the EU of being inflexible in applying the protocol's rules.

His comments echo longstanding complaints from Johnson, who suggested last year that the EU was being "theologically draconian" over the protocol's application.

The protocol resulted from Johnson's agreement struck with the EU in October 2019. This broke the long deadlock over Northern Ireland that had delayed Brexit itself.

It paved the way for the divorce deal with the EU, a general election victory, and finally the UK's exit from the EU in January 2020.

The prime minister's deal — he called it "excellent" at the time — did away with his predecessor Theresa May's ill-fated "backstop" measure, which his wing of the Conservative Party had strongly opposed, arguing it could have kept the UK in the EU for years.

But the new accord accepted Northern Ireland's special status compared to rest of the UK — and effectively created an Irish Sea border with Britain. The government's own impact assessment published that autumn outlined in detail the border checks that could be expected.

But over the ensuing weeks, Johnson repeatedly and wrongly asserted — examples are here and here — that there would be "no checks" on trade between Northern Ireland and Britain in either direction, contrary to the terms of the divorce deal he had just struck.

Ministers later accepted that the protocol did involve internal UK red tape.