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By Jayne McCormack
BBC News NI political correspondent
Unilateral action by the UK on the NI Protocol would mark a "historic low point", Taoiseach (Irish Prime Minister) Michéal Martin has said.
He made the remarks during a speech at the European Parliament on Wednesday.
It comes amid speculation that the government could publish legislation on Thursday which would allow UK ministers to override parts of the Northern Ireland Protocol.
Mr Martin accused Downing Street of having acted in "bad faith".
During Prime Minister's Questions in Westminster, Boris Johnson told MPs "the most important commitment that everybody in this House has made is to the balance and symmetry of the Belfast / Good Friday Agreement".
"That is our highest legal, international priority and it is what we must deliver."
What is the protocol?
The trade deal - the NI Protocol - governs how goods enter Northern Ireland from the rest of the UK and was agreed by the UK government and the European Union following the Brexit vote in 2019.
It was designed to prevent a hard border on the island of Ireland when the UK left the European Union.
The protocol led to the creation of new goods checks at Northern Ireland sea ports on some products from Great Britain, effectively creating a new trade border in the Irish Sea.
Unionist parties, including the DUP, argue that this has led to extra costs and unnecessary delays, as well as undermining the union between Northern Ireland and the rest of the UK.
A row over its impact has created a block on forming a devolved government in Northern Ireland, with the Democratic Unionist Party (DUP) blocking the establishment of the assembly since Sinn Féin emerged as the largest party in last month's election.
The DUP, which has the second highest number of Stormont seats, has refused to support the election of a new speaker or first and deputy first minister until there is "action" on the protocol.
'Reckless and damaging'
Speaking on Wednesday, Mr Martin said it was "perfectly reasonable" to want to make improvements to the protocol but warned that any attempt to do so outside the agreed joint mechanisms would damage the rule of law.
Sinn Féin deputy leader Michelle O'Neill said any action taken by the British government alone would be "unacceptable to the wider public and majority of MLAs who support the protocol".
"The threats by Boris Johnson to take unilateral action by legislating today, tomorrow or any day to breach an international agreement and undermine international rule of law is a dereliction of duty," she tweeted.
However, DUP MP Gavin Robinson said EU figures and the taoiseach were "quick to comment on perceived failures by the United Kingdom" but "remain entirely deaf to unionism and to the fact that progress in Northern Ireland requires both unionists and nationalists moving forward together".
Colum Eastwood, leader of the Social Democratic and Labour Party (SDLP) and Foyle MP, was critical of the UK government's approach to the protocol which he described as "a reckless and damaging course of action".
He said the government "claims to want to protect the Good Friday Agreement but in word and deed they have abandoned the politics of consensus", adding it had "ignored the clear will of people across the north".
Meanwhile, NI Minister Lord Caine said he hopes the new protocol bill will be published "very shortly" but could not commit to a time frame.
"Both houses [of Parliament] will take their scrutiny functions very seriously but it would be very foolish for people to try and frustrate, delay and hold this up because I think that would make the situation in NI worse than it is now," he told the BBC's Good Morning Ulster programme.
He added that it was widely acknowledged there were problems with the protocol and that the government was conscious an executive at Stormont would not be restored until it was dealt with.
Minister of State for Northern Ireland Conor Burns told Irish state broadcaster RTÉ that the government is "recalibrating, not tearing up, the implementation of the protocol".
He added: "We recognise the attractiveness of the protocol and the place that it leaves Northern Ireland in.
"But the reality is that we have now got ridiculously excessive checks on goods that are moving within the United Kingdom's internal market that will never go near the Irish Republic, that are absolutely no threat whatsoever to the integrity of the single market."
'Poking Boris Johnson in the eye'
Speaking in London on Tuesday, DUP MP Sammy Wilson told the BBC he felt there was "still some work to be done" on the government's new protocol bill.
He said he believed the government "has got the message" in terms of the "headline issues in the bill" but added that questions remained over how it will address his party's concerns "in the detail of the regulations".
The East Antrim representative voiced caution about rebel Conservative MPs who were considering opposing the forthcoming legislation as a way "to keep poking Boris Johnson in the eye" after the prime minister faced, and won, a vote of confidence by his own party.