The leaders of four pro-EU Stormont parties have written to the British government and EU urging commitments on NI in the Brexit deal to be kept.
It comes after Downing Street said it will bring a new law that could change post-Brexit customs plans with the EU.
Sinn Féin, the SDLP, Alliance and the Greens said the UK and EU must ensure the “rigorous implementation” of the NI protocol.
Number 10 said it will only make “minor clarifications in specific areas”.
Political parties at Stormont are divided over the government’s plans, and held an emergency debate to discuss it when assembly members returned to the chamber on Monday for the first time following summer recess.
The joint letter from the leaders of the four parties, which all supported remain in the EU referendum, was published on Monday afternoon.
It states that while the NI Protocol was “imperfect”, it guaranteed in all circumstances that there would be no hard border between Northern Ireland and the Republic of Ireland.
“It is entirely unacceptable to the Northern Ireland parties that the UK Government would seek to abandon these safeguards and mitigations, which we believe would amount to a serious betrayal of an existing international treaty,” the letter adds.
“The economic and political impact on the island of Ireland, on the UK and above all on the people whom we represent would be devastating, and long-lasting.
“It would represent a shocking act of bad faith that would critically undermine the Good Friday Agreement political framework and peace process and the UK’s ability to secure other crucial deals to protect the Northern Ireland economy.”
The four parties call on the UK government to “honour its commitments” and ensure the protocol is implemented rigorously.
What is the Northern Ireland Protocol?
The Northern Ireland part of the Brexit deal, known as the Protocol, was agreed in October last year and is due to come into effect at the end of this year.
It is designed to prevent a hard border in Ireland – or even any new checks at the Irish border.
It does this by effectively keeping Northern Ireland in the EU’s single market for goods.
This will mean products entering Northern Ireland from the rest of the UK will be subject to new checks and control – the so-called Irish Sea border.
However, the precise nature of these checks needs to be agreed by the EU and UK and are being negotiated in parallel with the trade talks.
It will also mean when relevant EU laws are amended or new ones are drawn up, they will also apply in Northern Ireland.
Under the plan, Northern Ireland would leave the EU customs union with the rest of the UK at the end of this year, but would continue to enforce the EU’s customs code at its ports.
Details on the nature and extent of goods checks at Northern Ireland ports are still to be agreed, ahead of the transition period ending on 31 December.
The government, however, denied that its plans would tear up the existing withdrawal agreement, arguing that it was “taking limited and reasonable steps” to clarify specific elements of the NI Protocol in domestic law.
Downing Street will not allow the UK’s internal market to “inadvertently be compromised by unintended consequences” of the Protocol, a spokesperson added.
Unionist parties in Northern Ireland are strongly opposed to the plan for Northern Ireland in the Brexit deal, fearing it damages the UK union.
‘NI question back on agenda’
The DUP’s chief whip at Westminster, Sammy Wilson, said he would “reserve judgement” until he saw the terms of any bill put forward this week.
Speaking on BBC Radio Ulster’s Talkback programme, the East Antrim MP said he wanted to see how the government would set out the detail.
“It would not be beyond the bounds of possibility that Northern Ireland is simply being cynically used in negotiations with the EU between now and the end of the year,” he said.
“Having been bitten once, we are not going to say that this is absolutely great, all I am saying is this, that the Northern Ireland question is now back on the agenda.
“It is back on the agenda because of the huge efforts we put in since January until now to spell out the consequences for Northern Ireland from the withdrawal agreement and also show how it spills over into policy for the rest of the United Kingdom.”
Ulster Unionist leader Steve Aiken said “the most important thing” for people in Northern Ireland was to consider the implications of the NI protocol.
“When it comes to state aid rules, or whether it comes to access to our supermarkets for goods coming from the rest of the country, if there is an opportunity to reconsider these elements in the protocol, now is the time,” he told BBC News NI.
However, Mr Aiken criticised the “mixed messages” coming from Westminster.
SDLP Infrastructure Minister Nichola Mallon has said “not enough clarity” is being provided to the executive from the British government.
She was taking questions in the assembly and was asked about the government’s plans to bring a law that could change the NI part of the deal agreed with the EU.
“If reports are true, this is entirely unacceptable. Any threat of a hard border must be resisted by the assembly and executive,” she said.
The executive meets weekly and also holds a regular sub-Brexit committee.
“It is clear to me, that not enough clarity is being provided to ministers from the British government and I’ll be pressing for answers,” said the minister.
Although the UK formally left the EU in January, it has continued to follow rules set in Brussels during a transition period – which ends on 31 December – while discussions over a long-term trade agreement continue.
Another round of talks – the eighth – begin on Tuesday, aimed at securing a deal to allow companies to trade without taxes or customs checks.
But Mr Johnson is expected to tell EU leaders it must be agreed in time for the European Council meeting on 15 October, if it is to be in force by 1 January.