Sinn Féin leader Gerry Adams has indicated his party may pull out of the power-sharing Stormont Executive.
It comes amid calls for the Northern Ireland First Minister Arlene Foster to step down during an inquiry into the controversial 'cash-for-ash' scheme.
Mrs Foster, who was elected First Minister a year ago, has been under pressure over the botched renewable energy scheme, which could cost taxpayers more than stg£400m (€539m).
A lack of cost controls meant businesses were receiving more in subsidies than they were paying for renewable fuel and the scheme was drastically oversubscribed.
Mrs Foster had said she will not be stepping down, and has described calls for her to do so as "misogynistic".
The Democratic Unionist leader also said she was ready for an election if Sinn Féin pulls out of the power-sharing government.
Opposition parties tabled a motion of no confidence in Mrs Foster before Christmas - which she survived.
But speaking at a meeting in Belfast on Saturday, Mr Adams said: "If the First Minister does not take the actions that society desires and deserves and which a sustainable process of change requires, then Sinn Féin will bring this ongoing and totally unacceptable state of affairs to an end.
"Today's meeting is part of us charting a way forward."
"The DUP leader has thus far refused to stand aside, without prejudice, pending a preliminary report by an Independent Investigation to the RHI scandal.
"She repeated that refusal yesterday. That is not good enough.
"Arlene Foster has been First Minister for almost a year. Martin McGuinness has proven his ability to work with the leaders of Unionism.
"But this can only be on the basis of partnership and equality. Martin told her that in a frank conversation on Wednesday.
"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 Féin allows it."