The DUP has blocked the election of a speaker in protest at the Northern Ireland Protocol, the agreement thrashed out between the UK government and Brussels to prevent a hard border with the republic of Ireland.
The stalemate – the Assembly unable to function – follows Sinn Fein topping last week’s Stormont elections and fears of an impasse.
The DUP’s leader, Jeffrey Donaldson, said he is sending a “clear message” to the EU and the UK government about resolving issues the Protocol.
Sinn Fein vice president Michelle O’Neill said accused the unionist party of “punishing the electorate” while Alliance Party leader Naomi Long said it had been a “shameful day” for the DUP.
The DUP is also refusing to nominate for the position of deputy first minister, which will prevent the forming of a new executive, as part of its protest against the protocol.
Unionists oppose the post-Brexit treaty because of the economic barriers it creates between Northern Ireland and the rest of the UK, and fears over the re-unification of Ireland, which is Sinn Fein’s over-arching aim.
It comes amid increased tensions between the Westminster government and the EU over the working of the protocol, which forms part of the UK’s Withdrawal Agreement with the EU.
Foreign secretary Liz Truss said the UK will have “no choice but to act” if the EU does not show enough “flexibility” over post-Brexit checks on goods going from Britain to Northern Ireland.
Prime minister Boris Johnson is set to visit Northern Ireland on Monday.