A group which includes representatives of loyalist paramilitaries has written to the prime minister to withdraw its support for the Good Friday Agreement.
The Loyalist Communities Council (LCC) called for changes to Brexit’s Northern Ireland Protocol.
The protocol means NI has remained in the EU single market for goods, so products arriving from GB undergo EU import procedures.
Unionists say it damages trade and threatens NI’s place in the UK.
In the letter, first reported by the Irish News, LCC chairman David Campbell said the group’s leadership is “determined that unionist opposition to the protocol should be peaceful and democratic”.
He added that the prime minister should not “underestimate the strength of feeling on this issue right across the unionist family.”
Mr Campbell said the LCC’s support for the Good Friday Agreement would be withdrawn “until our rights under the Agreement are restored and the protocol amended to ensure unfettered access for goods, services and citizens throughout the United Kingdom.”
The LCC represents groups including the Ulster Volunteer Force (UVF) and Ulster Defence Association (UDA) – loyalist paramilitary groups responsible for the murders of hundreds of people.
The Good Friday Agreement, also known as the Belfast Agreement, was signed in 1998 and marked the effective end of the Troubles in Northern Ireland.
It established a devolved power-sharing administration, and created new institutions for cross-border cooperation and structures for improved relations between the British and Irish governments.
It was approved by referendums in Northern Ireland and the Republic of Ireland in 1998 and was subsequently incorporated into British and Irish constitutional law and other areas of legislation.
The LCC letter states: “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 letter was published shortly after the EU said on Wednesday that a UK move to unilaterally extend grace periods for Irish Sea border checks as part of the NI Protocol would be a breach of international law.
In February Northern Ireland’s major unionist parties said they were supporting a legal bid to challenge the Protocol.
The Democratic Unionist Party (DUP), Ulster Unionist Party (UUP) and Traditional Unionist Voice (TUV) have said they are supporting judicial review proceedings.
In response to the letter, Alliance Party deputy leader Stephen Farry said Boris Johnson faced a decision over whether he would “give more oxygen to the normalisation of treating illegal organisations like any other stakeholder in society”.
The LCC was set up in 2015 by Mr Campbell, a former chairman of the Ulster Unionist Party, and Jonathan Powell, chief of staff to former Prime Minister Tony Blair, as a means to help bring an end to loyalist paramilitarism.
It is not a proscribed organisation and is said to represent a range of individuals and views from the wider Protestant, unionist and loyalist communities.
In late February the DUP leadership held talks with the organisation to discuss opposition to the Protocol.
It came shortly after Mr Campbell was criticised after saying loyalists would “fight physically” to maintain “freedoms” within the UK.
In January, the LCC also met top civil servants from the Northern Ireland Office (NIO) to express anger at the Irish Sea border arrangements.