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By Mark Simpson
BBC News NI
The Stormont Assembly will meet at noon as the deadline looms for restoring the Northern Ireland executive.
The executive is made up of ministers from the largest parties, and is designed to ensure unionists and nationalists govern together.
The DUP is stopping it being formed as part of its ongoing protest against the Northern Ireland Protocol.
If it does not come back by midnight, the Northern Ireland secretary has said he will call a fresh election.
Sinn Féin and the Alliance Party have backed a motion to bring back assembly members for a special sitting and are calling on the Democratic Unionist Party (DUP) to support the election of an assembly Speaker and stop blocking the formation of an executive.
A political vacuum looms, with no indication the political deadlock will be broken before Friday's 00:01 BST deadline - 24 weeks since May's election.
Caretaker ministers at Stormont will be removed from office and senior civil servants will be in charge.
The assembly - which has been meeting only for special recalls since the election - would also be dissolved.
Mr Heaton-Harris, who has been in office since 6 September, has consistently said in recent weeks he will call the election, rather than try to delay it or avoid it with fresh legislation at Westminster.
The most likely date for a poll is 15 December.
A decision on this could come quickly, perhaps as soon as Friday.
Ministers have been in post, but with only limited powers, since the DUP withdrew from the executive in February.
This is because DUP, which is the largest unionist party at Stormont, has also blocked the election of an assembly speaker as part of its anti-protocol protests.
The protocol is the trading arrangement, negotiated during the Brexit talks, which means goods coming into Northern Ireland from Great Britain are subject to checks.
What happens if the election deadline is missed?
The rules state that the Northern Ireland secretary must call an election "as soon as is practicable".
That does not mean he must call an election on 28 October, contrary to what he has repeatedly said.
The rules do say the election must be held within 12 weeks, which would mean the second assembly election in the space of a year.
However previous deadlines in Northern Ireland have been adjusted by emergency legislation at Westminster.
The recall of the assembly later is likely to offer the parties an opportunity to outline their competing positions on the current stalemate.
In theory, they are due to discuss a motion on the cost-of-living crisis, but the likelihood is that procedural rules mean it will not be debated.
In a letter to assembly members, Speaker Alex Maskey told them: "If the assembly is unable to elect a speaker and deputy speakers, it cannot proceed to do any of the other business including the appointment of ministers and the debate on the motion."
The DUP - as well as the Ulster Unionist Party and Traditional Unionist Voice - say that the protocol is undermining Northern Ireland's place in the UK and is contrary to the spirit of the 1998 Good Friday Agreement, the peace deal which set up power sharing in Northern Ireland.
It keeps Northern Ireland aligned with some EU trade rules to ensure goods can move freely across the Irish land border, from the UK into an EU member state, the Republic of Ireland.
It created a new trade border between Northern Ireland and the rest of the UK, something the EU has accepted is causing difficulties for many businesses.
If the executive does not return, Sinn Féin vice-president Michelle O'Neill has called for a "joint approach" between London and Dublin.