LEICESTER, United Kingdom – Loyalist paramilitary groups in Northern Ireland say they are “withdrawing support” from the 1998 Good Friday Agreement that brought an end to “The Troubles” – a 30-year conflict that left over 3,500 people dead.

In a letter to UK Prime Minister Boris Johnson, the Loyalist Communities Council (LCC) said it objected to the NI Ireland protocol, part of the Brexit agreement between the United Kingdom and the European Union which creates a customs border between Northern Ireland and the rest of the UK. The protocol was negotiated to keep an open border between Northern Ireland and the Republic of Ireland, a key feature of the Good Friday Agreement.

Much of the 1998 peace accord was undergirded by Ireland and the UK’s common membership in the European Union, and the Brexit process has shaken the agreement to its core.

The UK has already caused consternation in its European partners by unilaterally extending a “grace period” keeping a free flow of goods across the Irish Sea until October.

The period was due to end at the end of March, and once expired will require extra paperwork to ship goods between Northern Ireland and the other parts of the UK.

The Loyalist paramilitaries are predominantly Protestant and support the union of Great Britain and Northern Ireland, and waged a low-level conflict against the Irish Republican Army and other Nationalist paramilitaries that are predominantly Catholic and support a united Ireland. Both sides agreed to lay down their arms as part of the Good Friday Agreement.

The letter from the LCC, first reported by the Irish News, said the Northern Irish protocol “should be replaced,” and that an Irish Sea border would not be acceptable.

“The LCC is prepared to play a meaningful role in seeking a workable solution, however, a starting point has to be the acceptance that a hard border on the island of Ireland, or between Northern Ireland and the rest of the United Kingdom, has no cross-community support here and therefore untenable,” says the letter to the Prime Minister.

The LCC said the terms of the Brexit treaty violate the terms of the Good Friday Agreement, and all the communities in Northern Ireland are opposed to it.

“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 group warned.

Jeffrey Donaldson, a member of the UK Parliament from the Democratic Unionist Party (DUP), told the BBC the letter doesn’t mean loyalist paramilitaries would return to violence.

“Of course we are very clear that choosing the path of violence is not the way to go in any circumstances,” he said.

The DUP has been criticized for meeting with members of the LCC to discuss the protocol, since the group includes outlawed organizations such as the Ulster Volunteer Force, Ulster Defence Association and Red Hand Commando, which were behind several murders during “The Troubles.”

“Ignoring communities will take Northern Ireland in the wrong direction,” Northern Ireland First Minister and DUP leader Arlene Foster told the Irish News, defending the meetings.

Follow Charles Collins on Twitter: @CharlesinRome