Four rival unionist parties in Northern Ireland have formed an alliance to fight the Brexit protocol, issuing a joint statement weeks after the Democratic Unionist party threatened to quit Stormont if it was not scrapped.

It is seen as a significant attempt to show that the DUP’s hardline position is not isolated before the expected publication by the EU of fresh proposals to address UK demands to substantially rewrite the protocol.

In their statement, they warn of the “grave damage” inflicted by the protocol to Northern Ireland and say good relations with Ireland and the EU cannot be achieved as long as the region is subjected to EU laws, the bedrock of the protocol.

It was signed by the DUP, the Ulster Unionist party, the Traditional Unionist Voice and the Progressive Unionist party.

“The huge disruption of trade in the supply of goods from Great Britain to Northern Ireland has caused unnecessary supply chain disruption and unacceptable and unsustainable levels of bureaucracy and barriers to trade within our own nation.

“The resulting diversion and reorientation of trade is destructive of Northern Ireland’s place in the United Kingdom and will result in an economic realignment which is unacceptable,” they said.

The DUP, whose popularity has declined in the polls, threatened to quit the Stormont executive earlier this month unless there were substantial changes to the protocol.

It said it would do so “within weeks” and was condemned by rival parties, including some on the unionist side.

The protocol has been suspended in parts after the UK unilaterally decided to extend a grace period for checks on goods crossing the Irish Sea from Great Britain to Northern Ireland.

But the EU said it would not take retaliatory action and is expected to present fresh proposals to meet the UK’s demands after the Conservative party conference next week.

The four parties warned they could not support the protocol unless the EU and UK scrapped all checks on goods, effectively dismantling the arrangements. It called for “a proportionate and equitable solution” to restore “unfettered trade” in the UK’s internal market.

Some see the DUP’s stance as an attempt to position itself for Stormont elections after a turbulent summer that included three changes at the top of the party. Sir Jeffrey Donaldson, the current leader, has made a series of high-profile media appearances in the past two weeks, including writing to the speaker of the US House of Representatives, Nancy Pelosi, enclosing a copy of the Good Friday peace accord.