Sinn Féin leader Gerry Adams has said his party will not back down from its demand for First Minister Arlene Foster to stand aside to allow an inquiry into her role in a botched green energy scheme.
Ms Foster has voiced support for a judge-led inquiry, outlined by Sinn Féin, into the controversial Renewable Heat Incentive (RHI) .
However, she has rejected a prerequisite from Sinn Féin that she temporarily step aside to facilitate it.
Ms Foster presided over the development of the error-ridden scheme during her time as economy minister, which has left the power-sharing administration facing a £490m bill.
Speaking at a party meeting in Belfast today, Mr Adams said if Ms Foster does not take the actions that "society desires and deserves", then Sinn Féin will bring the "ongoing and totally unacceptable state of affairs to an end".
He warned the Democratic Unionist leader that she is not a "Prime Minister" and her position in one of the two leading roles in the power-sharing administration depended on his party's agreement.
The state-funded RHI was supposed to offer a proportion of the cost businesses had to pay to run eco-friendly boilers, but the subsidy tariffs were set too high, and without a cap, it ended up paying out significantly more than the price of fuel.
This enabled applicants to "burn to earn" - getting free heat and making a profit as they did it.
Claims of widespread abuse include a farmer allegedly set to pocket around £1m in the next two decades for heating an empty shed.
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While Ms Foster's statement yesterday evening adopted a somewhat more conciliatory tone in comparison to recent vexed exchanges with Sinn Féin over the scandal, the First Minister made clear she will not give ground on what is one of their key demands.
Ms Foster accused political opponents of wanting to be her "judge, jury and executioner".
The lack of consensus over the form of an investigation comes amid similar disagreement on DUP proposals to reduce the overspend. The DUP has claimed its proposals could wipe out the bill, but Sinn Féin has rubbished them.
In his address, Mr Adams said: "The DUP leader has thus far refused to stand aside, without prejudice, pending a preliminary report by an independent investigation into the RHI scandal.
"She repeated that refusal yesterday.
"That is not good enough. Arlene Foster has been First Minister for almost a year."
He added: "Arlene Foster is not a Prime Minister. She is a co-equal partner in the Office of First and Deputy First Minister. She can continue in that office but only for as long as Sinn Fein allows it."
In a strongly-worded speech he also branded DUP Communities Minister Paul Givan an "ignoramus" for cutting funding for an Irish language initiative just before Christmas.
He said during ten years of partnership government together, the DUP had shown Sinn Féin "deliberate provocation, arrogance and disrespect".
The total RHI spend in Northern Ireland is estimated at £1,150m over the next 20 years.
The Treasury is set to cover £660m of that, with Stormont landed with the remaining £490m.
In another development, Sinn Féin announced it was lodging a motion of no confidence in the Assembly's DUP speaker.
Robin Newton has been under political pressure over his handling of a recalled Assembly session to debate RHI before Christmas.
He has also been forced to defend himself against conflict of interest accusations in regard to his handling of Assembly exchanges on a controversial charity in his east Belfast constituency.
Sinn Féin's Declan Kearney said: "He is now part of the problem and an integral part of the unfolding crisis and he should resign immediately."