A majority of Northern Ireland's lawmakers have told Boris Johnson they oppose his plan to amend the Northern Ireland Protocol.
The PM has defended controversial legislation, set to be tabled in the Commons later on Monday, which will allow ministers to override aspects of the agreement.
He claimed it is "not a big deal" to "fix" the protocol, a deal he agreed with the EU, despite claims he is about to break international law by changing it.
But a letter signed by 52 of Northern Ireland's 90 MLAs (Members of the Legislative Assembly), told him Sinn Fein, the SDLP and the Alliance Party all oppose his plans.
"We reject in the strongest possible terms your government's reckless new protocol legislation, which flies in the face of the expressed wishes of not just most businesses, but most people in Northern Ireland".
The letter, shared online by Sinn Fein Vice president Michelle O'Neill, continues that "whilst not ideal, the protocol currently represents the only available to Northern Ireland from the worst impacts" of Brexit.
Mr Johnson says his main priority is protecting the Good Friday Agreement which helps ensure peace in Northern Ireland.
The Northern Ireland Protocol, agreed jointly between the UK and EU as part of the Brexit Withdrawal Agreement, is designed to allow free flowing trade on the island of Ireland and prevent a hard border.
But it is impacting trade between Great Britain and Northern Ireland due to the checks, intended to protect the EU's single market, being carried out on products flowing east to west.
Unionists in Northern Ireland say the checks are effectively separating their country from the rest of the UK, undermining the Good Friday Agreement - which keeps NI part of the union - and for that reason the Democratic Unionist Party is refusing to enter government until issues are resolved.
Prime Minister Johnson says the protocol must be amended to help get a government up and running in Northern Ireland, but the European Union is threatening a trade war if the PM decides to rip up the deal he personally signed.
Speaking to broadcasters on a farm in Cornwall, the prime minister said: "We've got a problem at the moment, which is in Northern Ireland, the Stormont assembly, the government of Northern Ireland, can't meet because of the effects of the protocol. What it does is it creates unnecessary barriers on trade east-west.
"What we can do is fix that. It's not a big deal, we can fix it in such a way as to remove those bureaucratic barriers but without putting up barriers on trade moving north-south in the island of Ireland as well."
Irish premier Micheal Martin has said it is regrettable that the UK is to renege on an international treaty, after the British government insisted it's move was "lawful" and "correct".
Speaking in Cork on Monday, Mr Martin said he rejects the assertions from the British Government that the EU has not been flexible during negotiations.
"It's very regrettable for a country like the UK to renege on an international treaty," he said.
"I think it represents a new low point because the natural expectation of democratic countries like ourselves, the UK and all across Europe is that we honour international agreements that we enter into."
Following a 12-minute phone call between Irish Foreign Affairs Minister Simon Coveney and Ms Truss, a spokesperson for Ireland's Department of Foreign Affairs said he feels the move by the UK to publish the legislation, "is deeply damaging to relationships on these islands and between the UK and EU".
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"Mr Coveney said it marks a particular low point in the UK's approach to Brexit, especially as Ms Truss has not engaged with negotiations with the EU in any meaningful way since February."
The spokesperson added: "The UK's unilateral approach is not in the best interest of Northern Ireland and does not have the consent or support of the majority of people or business in Northern Ireland.
"Far from fixing problems, this legislation will create a whole new set of uncertainties and damage relationships."
Mr Coveney tweeted: "UK Govt now proposing to set aside Int Law, reject a partnership approach, ignore majority in NI & deliberately ratchet up tension with an EU seeking compromise.
"We remain open to dialogue to find agreement but his approach adds to instability & is no fix."
The Bill due to come before Parliament will see the government move without the consent of the EU to change the terms of the protocol in a bid to reduce the checks on the movement of goods across the Irish Sea.
This could include allowing ministers to remove all customs processes for goods moving within the United Kingdom and enable the frictionless movement of agri-food goods staying within the UK.
It could also see businesses in Northern Ireland given the ability to choose whether to follow UK or EU regulations, depending on who they are trading with.