Deputy first Minister Martin McGuinness has called for DUP leader Arlene Foster to stand aside from her role as First Minister while a full investigation is carried out into Stormont's flawed Renewable Heating Incentive scheme, as the political crisis escalates.
It comes after Mrs Foster's former ministerial colleague, Jonathan Bell, broke party ranks to make a series of explosive allegations to the BBC on Thursday night.
Mr Bell claimed DUP advisers attempted to delay the closure of the scheme, which could cost Stormont an estimated £400m.
He said that his party leader should apologise to the people of Northern Ireland for her handling of the so-called 'Cash for Ash' scandal.
The DUP leader has strenuously denied his claims and has accused the former Enterprise Minister of acting aggressively towards her in a stormy meeting earlier this year.
She claimed that Mr Bell was making the allegations to mask his own failings over the controversial green energy scheme - set up by Mrs Foster - which paid out subsidies well in excess of the costs of buying renewable fuel.
Speaking on Friday afternoon, Mr McGuinness called for an "independent investigation".
“It is my belief the only way to establish the truth, and rebuild the reputation of the institutions, is to urgently establish a fully independent investigation into this matter," the Sinn Fein MLA said.
Mr McGuinness said he "spoke by phone this afternoon with the First Minister".
“I outlined my serious concern that the credibility of the political institutions is being undermined by the serious and ongoing allegations surrounding the design, operation, abuse and ending of the Renewable Heating Incentive Scheme", he said.
“This includes allegations from a former DUP Minister that there was corruption.
“This scheme has directly impacted on the public purse. Taxpayers’ money wasted in this scheme, needs to be retrieved."
Mr McGuinness said he told Mrs Foster that "in the public interest, she should stand aside from the role as First Minister while that investigation is underway and at least until an initial assessment had been concluded into the veracity of all the allegations."
“That is what I would do if I was in this situation. I asked the First Minister to take the time and consider this suggestion carefully,” he added.
Responding, a DUP spokesman rejected the call for Mrs Foster to stand aside.
"The First Minister will not be stepping aside, but instead is focused on ensuring the full facts about this issue emerge and proposals are brought forward which can make a significant reduction in the future financial burden the Executive would face," a statement read.
"The First Minister does not take her instructions from Sinn Fein, but from the electorate.”
Earlier Mrs Foster admitted it had been a "tough old week".
She also told the BBC: "A mark of a politician isn’t in good times, it’s in challenging times. I am determined to see this through. I am determined, whilst others walk away, to make sure that we find a solution to all of this because I owe it to the people of Northern Ireland and I take my duties and responsibilities as First Minister very, very seriously.”
She faces a motion of no confidence on Monday as the devolved assembly is recalled for a special sitting to discuss the growing political crisis in the run-up to Christmas.
The scheme is expected to cost taxpayers in Northern Ireland £400m amid claims of widespread abuse, including a farmer apparently set to pocket around £1m in the next 20 years for heating an empty shed.
DUP First Minister Arlene Foster over-ruled her own minister from immediately shutting down the Renewable Heating Incentive (RHI) scheme which could cost the tax-payer an estimated £400m, it was claimed last night.