Boris Johnson will later publish plans to scrap parts of the post-Brexit deal he struck with the EU in 2020.
The PM wants to change the Northern Ireland Protocol to make it easier for some goods to move between Britain and Northern Ireland.
But the EU is against the move, saying it would break international law.
The UK insists its proposals – to be set out in a parliamentary bill – are legal and will secure the future of the United Kingdom.
The two sides agreed the protocol as they strived to maintain peace in Ireland – including the open Irish border – following Brexit.
Fifty two members of the 90 strong Northern Ireland Assembly, including those from Sinn Féin, the Alliance and the SDLP, have written to the prime minister to say they “reject in the strongest possible terms your government’s reckless new protocol legislation”. No unionist assembly members signed the letter.
Irish Prime Minister Micheál Martin has called for negotiations between the UK and EU to deal with the impasse.
He described the UK government’s decision to change the protocol as a “low point”, which was “very regrettable”.
The Republic of Ireland is still part of the EU’s single market for goods and services, which the UK has left.
Rather than impose a hard border – involving checks on goods and people moving between Northern Ireland and the Irish Republic – the protocol introduces checks on some goods travelling from Great Britain (England, Scotland and Wales) to Northern Ireland.
The Democratic Unionist Party (DUP), which won the second-most seats in the recent Northern Ireland Assembly elections, argues this creates a divide that could lead to the break-up of the UK.
It is refusing to set up a new ruling Northern Ireland executive with Sinn Féin, which won most seats in the elections and accepts the protocol, until changes are made to the wording.
The UK government, despite signing up to the agreement in January 2020, has also raised concerns, prompting the publication of its bill later on Monday.
Foreign Secretary Liz Truss said she wanted to “fix the problems with the Northern Ireland Protocol and restore political stability”, adding that Brussels must be “willing to change”.
But European Commission Vice-President Maros Šefčovič tweeted that the UK’s actions were “damaging to mutual trust and a formula for uncertainty”.
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Johnson to unveil plans to scrap parts of NI Protocol agreed with EU
