Northern Ireland's influential Orange Order said it had voted unanimously to oppose any return of devolved government in the British province unless substantial improvements are made to a reworked post-Brexit deal with the European Union.
The move by the 30,000-member pro-British group - which holds large, at times divisive marches each year to celebrate centuries-old military victories - could raise pressure on the Democratic Unionist Party to wring some concessions from London.
The DUP has said it will not drop a year-long boycott of the devolved assembly without "further clarification, re-working and change" of the Windsor Framework, agreed by the British government with the EU to resolve disputes over trade rules in Northern Ireland following Britain's withdrawal from the EU.
Prime Minister Rishi Sunak's government, keen to improve post-Brexit relations with the EU, insists the deal is not open for renegotiation.
"The Windsor Framework has delivered some limited, but welcome practical adjustments to the existing protocol. However, it does not resolve the fundamental concerns," the Orange Order said in a statement on Tuesday.
"Many aspects of the Windsor Framework have been oversold. The Windsor Framework continues to treat Northern Ireland as a place apart within the United Kingdom and equal citizenship has not been restored."
The statement said the Orange Order would not endorse the deal without "substantial and tangible progress which resolves these fundamental issues".
The DUP, at odds with opinion polls that say most Northern Irish voters support the deal, has raised concerns over the continued role of EU law and Northern Ireland's place in the United Kingdom's internal market.
It opposed a key element of the deal in a vote last week that won the overwhelming backing of the British parliament but has said it may yet be convinced to support the agreement if additional concessions are made.
A group of mainly current and former DUP members is due to report back to the party this week on a month-long consultation.
The Orange Order similarly raised "fundamental concern" that Northern Ireland citizens remain subject to EU laws and that the British-run region "continues to be a semi-detached part" of the UK's economic union.
As part of Brexit, Northern Ireland effectively remained in the bloc's single market to avoid a hard border with EU-member Ireland. The resultant internal trade boundary with the rest of the UK has angered many pro-British unionists.
The Orange Order said London must introduce new legislation to create fully frictionless trade between Britain and Northern Ireland, and make businesses opt-in to EU laws if they wish to trade in the EU single market.
Business groups have overwhelmingly supported the Windsor Framework, seeing it as removing damaging uncertainties over trading relationships.
(Reporting by Amanda Ferguson, writing by Padraic Halpin, editing by Mark Heinrich)
