An umbrella organisation representing loyalist groups, including the UDA, UVF and Red Hand Commando, has written to two MPs and 10 MLAs warning that "no form of Irish Sea border will ever be tolerated".
The newly formed Ards and North Down Unionist Loyalist Collective speaks of "growing sentiment against all aspects of the peace process, should our community be forced down this path against our will".
It has sent an open letter to Alliance North Down MP Stephen Farry and DUP Strangford MP Jim Shannon as well as the 10 MLAs representing both constituencies.
The correspondence has the support of senior loyalist figures including North Down UDA commander David Stitt.
The group told the politicians it anticipates "an urgent response setting out your position as to steps you intend to take to represent the views of your constituents".
Mr Farry said that the letter had been emailed to him by the Ards and North Down Unionist Loyalist Collective.
"I am happy to engage with constituents regarding the protocol, but I don't engage with people who hide behind artificial titles and are anonymous," he added.
The letter stated: "It is a matter of deep and profound concern for those under our umbrella that an Irish Sea border, imposing an economic united Ireland upon our community, is being forced on us against our will.
"This is even more concerning given that the imposition of this internal UK border has been put upon us as a reward for republican threats of violence.
"We are deeply concerned, and are repeatedly hearing from those at a grassroots level, in relation to the worrying precedent this has set."
The group acknowledged that the "practical day-to-day effects" of the Irish Sea border - which include increased paperwork and checks on importing and exporting goods - could potentially be successfully addressed.
It focused attention on the ideological implications of the protocol - the mechanism which sees Northern Ireland follow EU rules on certain goods, thus avoiding a hard border on the island of Ireland - which it said could never be mitigated.
The letter continued: "It is for this reason we represent to you, as an elected representative, the clear and robust view of those under our umbrella - within the area we represent - that no form of Irish Sea border will ever be tolerated."
The letter claimed that the cross-community consent mechanism in the Good Friday Agreement had been "shredded".
It said: "Our collective shares the growing sentiment across the unionist/loyalist family that it is illogical and inconceivable that political unionism should continue to operate the very institutions which are designed to act as a pathway to a united Ireland. In this vein, we ask you as a local representative for your commitment to taking all steps necessary to resist the imposition of an economic united Ireland, which places us on the window ledge of the Union.
"Support from within our community for the Belfast Agreement was predicated upon the understanding that the Union was safe. This can no longer with any credibility be said to be true.
"It is natural therefore that there will be growing sentiment against all aspects of the peace process, should our community be forced down this path against our will."