Loyalist paramilitary organisations have told British Prime Minister Boris Johnson they are withdrawing support for the Good Friday Agreement.
The groups said they were temporarily withdrawing their backing of the peace accord amid mounting concerns about the Northern Ireland Protocol.
However, the Loyalist Communities Council stressed that unionist opposition to the protocol should remain "peaceful and democratic".
A DUP MP has also said that the move does not signal an intent to return to violence.
Jeffrey Donaldson, whose party leadership recently met an umbrella group representing the outlawed paramilitary groups, said there was no evidence the groups planned to resist the protocol by violent means.
He told BBC Radio Ulster that his party colleagues came away from their meeting with the LCC with "no sense that loyalist paramilitaries were going to revert to violence in opposition to the Northern Ireland Protocol".
"I think that is reflected in this statement," he added.
"Of course we are very clear that choosing the path of violence is not the way to go in any circumstances."
The 1998 Good Friday Agreement that loyalist paramilitaries endorsed 23 years ago ended decades of violence and established devolved power-sharing at Stormont.
British MPs are facing a backlash from unionists who fear the post-Brexit protocol threatens Northern Ireland's place in the UK internal market.
The DUP and other unionist parties are pushing for the protocol to be ditched, claiming it has driven an economic wedge between Northern Ireland and Britain which undermines the union.
A letter sent to Mr Johnson by an umbrella body representing the paramilitaries said their stance in respect of the Good Friday Agreement would continue until the protocol was amended to ensure "unfettered access for goods, services, and citizens throughout the United Kingdom".
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It added: "If you or the EU is not prepared to honour the entirety of the agreement then you will be responsible for the permanent destruction of the agreement."
The development came as the UK government took unilateral action yesterday to extend a grace period that has been limiting the volume of red tape associated with moving agri-food goods from GB into Northern Ireland.
The EU has criticised the British government move to extend the exemption period, which was due to run out at the end of the month, until October, claiming it risked breaching the terms of the protocol.
The letter to Mr Johnson was written by David Campbell, chairman of the LCC. Mr Campbell has penned a similar letter to Taoiseach Micheál Martin.
The LCC represents the Ulster Volunteer Force, Ulster Defence Association and Red Hand Commando, which were responsible for many deaths during the Troubles.
The main loyalist and republican armed groups signed up to principles such as commitment to non-violence during discussions, which led to the signing of the Good Friday Agreement in exchange for early release of prisoners.
The letter states: "We are concerned about the disruption to trade and commerce between Northern Ireland and the rest of the United Kingdom that is occurring, but our core objection is much more fundamental."
It said during the Brexit negotiations the British government and the EU said it was paramount to protect the agreement and its built-in safeguards for the two main communities in Northern Ireland.
The letter said the operation of the protocol "repeatedly breaches those objectives".
Mr Campbell insisted the LCC leadership is determined that opposition to the protocol should be "peaceful and democratic".
"However, please do not under-estimate the strength of feeling on this issue right across the unionist family," he adds.
"The only time I can remember such unanimity of opposition was following the imposition of the Anglo-Irish Agreement in 1985.
"Accordingly, I have been instructed to advise you that the loyalist groupings are herewith withdrawing their support for the Belfast Agreement until our rights under the agreement are restored and the protocol is amended to ensure unfettered access for goods, services, and citizens throughout the United Kingdom.
"If you or the EU is not prepared to honour the entirety of the agreement then you will be responsible for the permanent destruction of the agreement."
Police have noted growing discontent in unionist communities.
PSNI Chief Constable Simon Byrne previously warned of a "febrile" atmosphere and urged people to step back from the brink of violence.
Inspection staff at ports were temporarily withdrawn from duties earlier this year in response to sinister graffiti, but they later resumed their work after police insisted their was no credible threat against them.
Last week, Stormont's DUP Agriculture Minister Gordon Lyons stopped preparatory work on building permanent Irish Sea trade checks at the ports.
That move, the legality of which has been disputed by executive colleagues, did not impact ongoing checks, as those are happening at temporary port facilities.
PSNI probes 'threat' to UK minister over Brexit fallout
The PSNI is investigating graffiti in a loyalist area of Belfast referencing UK Cabinet Office Minister Michael Gove.
The message painted on a wall in Sandy Row included Mr Gove's name and an address.
It is the latest example of anti-Northern Ireland Protocol graffiti that has appeared in loyalist areas since the new Irish Sea trading arrangements came into operation.
Mr Gove has co-chaired the joint EU/UK committee responsible for implementing the protocol.
Asked about the incident, the PSNI said: "We do not discuss the security of individuals and no inference should be drawn from this. However, we would never ignore something that could put an individual at risk."
Mr Johnson's official spokesman told reporters in London that the PSNI is "responding to this situation".