Unionists in Northern Ireland argue that the Northern Ireland Protocol "fundamentally alters" the Belfast Agreement, also known as the Good Friday Agreement.

Four Unionist leaders in Northern Ireland have signed their names to a joint declaration to "affirm our opposition to the Northern Ireland Protocol, its mechanisms and structures and reaffirm our unalterable position that the Protocol must be rejected and replaced by arrangements which fully respect Northern Ireland’s position as a constituent and integral part of the United Kingdom”.

The brief statement, issued on September 28, is co-signed by Sir Jeffrey Donaldson MP, Democratic Unionist Party (DUP) Leader; Doug Beattie MC MLA, Ulster Unionist Party (UUP) Leader; Jim Allister MLA Traditional Unionist Voice (TUV) Leader; Cllr Billy Hutchinson, Progressive Unionist Party (PUP) Leader.

The four leaders also recorded this video:

An explanatory note about Tuesday's joint declaration says in part: "The Northern Ireland Protocol fundamentally alters the arrangements within the Belfast Agreement by making Northern Ireland subject to European Union laws and processes of a Single Market for goods, under a European Union customs code and VAT regime, with the rest of our nation decreed a 'third country' when it comes to trade and the import of goods."

It adds: “Through the Northern Ireland Protocol, the European Union asserts sovereignty over economic and trading matters in Northern Ireland, subjecting us to their laws - which we cannot change - and the rule of the European Court of Justice without local political representation, creating a major democratic deficit.

“We wish nothing more than good and practical relations with the European Union and the Irish Republic which is our nearest neighbour and with whom we share a frontier between the United Kingdom and the European Union. However, this cannot be achieved with Great Britain designated as a ‘third country’, with a regulatory border partitioning the United Kingdom, and subjecting Northern Ireland to European Union laws and jurisdiction.

“Thus, going forward, any agreement which fails to ensure a proportionate and equitable solution which respects the sovereignty of the United Kingdom and restores our unfettered place within the Internal Market, cannot command the support of the unionist community.”

The Northern Ireland Protocol, a portion of the agreed-upon Brexit arrangement, aims to keep Northern Ireland in both the United Kingdom's customs territory and the EU's single market. It effectively creates a border in the Irish Sea, much to the anger of unionists.

In July, the UK government published a 'command paper' on the Northern Ireland Protocol in which it lays out new proposals for "the way forward." However, EU officials said afterward that they will not renegotiate on the Northern Ireland Protocol but will "continue to be creative and flexible within" its framework.

The tension surrounding the Northern Ireland Protocol has also garnered attention from high-level US politicians, who have warned that if the Good Friday Agreement is threatened, it could have implications on a new US-UK trade deal.