The British government has published plans to get rid of parts of the post-Brexit deal it signed with the EU in 2020.
It wants to change the Northern Ireland Protocol to make it easier for some goods to flow from Great Britain to Northern Ireland.
But the EU opposes the move, saying that going back on the deal breaches international law.
The UK government disputes this, arguing that the changes will mean the United Kingdom stays together.
The alterations are set out in the Northern Ireland Protocol Bill, to be debated and voted on by Parliament.
The government is promising to remove “unnecessary” paperwork on goods checks and that businesses in Northern Ireland will get the same tax breaks as those elsewhere in the UK.
The bill will also ensure that any trade disputes are resolved by “independent arbitration” and not by the European Court of Justice, it adds.
Foreign Secretary Liz Truss said it was “a reasonable, practical solution to the problems facing Northern Ireland” and that the UK could “only make progress through negotiations if the EU are willing to change the protocol itself”, adding: “At the moment they aren’t.”
“We are very clear that we’re acting in line with the law,” she said.
The British government said it would prefer a “negotiated solution” with the EU that avoids the need for the bill to become law.
But Sinn Fein’s Stormont leader Michelle O’Neill accused the prime minister of creating more instability and uncertainty in Northern Ireland.
“Boris Johnson’s action is illegal, he is in clear breach of international law, regardless of the detail,” she said.
“He himself signed up to an agreement, he signed on the dotted line and he’s now legislating to breach that international agreement.”
She added that the protocol was working – and she accused the DUP of blocking the formation of an Executive in the North of Ireland.
Sinn Féin won the most seats in the recent assembly elections.
The DUP, which won the second-highest number of seats, has said it will not form a power-sharing executive with its rival party until the protocol is amended.
It argues that as it stands, the rules create a divide that could lead to the break-up of the UK.
DUP leader Sir Jeffrey Donaldson said: “I don’t believe what the government is proposing is illegal. I believe it is a solution and that is what we need – solutions.”
He added: “We have strong support from across unionism for the stand that we are taking.
“I believe that our pressure is seeing progress being made and we will continue to work with government to ensure that this legislation progresses.”
European Commission Vice-President Maros Šefčovič tweeted that the UK’s actions were “damaging to mutual trust and a formula for uncertainty”.
And Irish Prime Minister Micheál Martin called for negotiations between the UK and EU to deal with the impasse.
The EU could announce it is launching legal action against the UK for alleged breaches of the protocol as soon as Wednesday, the BBC understands.
This would involve resuming infringement proceedings that were paused last year, over the UK’s unilateral extension of grace periods.
There could also be new infringement proceedings over claims the UK’s failed to meet its obligations on border control posts and data sharing.
In legal notes published alongside the bill, the UK government says: “This is a genuinely exceptional situation, and it is only in the challenging, complex and unique circumstances of Northern Ireland, that the government has, reluctantly, decided to introduce legislative measures which, on entry into force, envisage the non-performance of certain obligations.”
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