Martin McGuinness has called on Arlene Foster to "step aside"

Northern Ireland's power-sharing government has been plunged into crisis as First Minister Arlene Foster flatly rejected Deputy First Minister Martin McGuinness's call for her to "stand aside" to allow a probe into corruption allegations.

The row threatening the region's peace-building institutions has erupted over a controversial green energy scheme - set up by Mrs Foster - which paid out subsidies well in excess of the costs of buying renewable fuel.

It is expected to cost taxpayers in Northern Ireland £400 million amid claims of widespread abuse of the scheme including a farmer apparently set to pocket around £1 million in the next 20 years for heating an empty shed.

The "cash for ash" scandal reached fever pitch on Thursday when former Democratic Unionist Party economy minister Jonathan Bell broke ranks to level a series of explosive claims against his leader Mrs Foster and party advisers.

In an extraordinary TV interview, a tearful Mr Bell said God told him to come clean as he claimed a "highly agitated and angry" Mrs Foster demanded he keep the Renewable Heat Initiative open for an extra fortnight despite its huge losses.

Against the backdrop of street protests in Belfast and Londonderry calling for Mrs Foster to go, Mr McGuinness telephoned Mrs Foster and asked her to temporarily "stand aside" in the public interest to allow an independent investigation into the affair.

The Sinn Fein chief said he told her he was seriously concerned the credibility of Northern Ireland's political institutions is being "undermined" by the allegations.

"This includes allegations from a former DUP Minster that there was corruption," he said.

But the DUP quickly let it be known that Mrs Foster had no intention of going anywhere - adding she didn't take instructions from her power-sharing partners Sinn Fein.

"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 party spokesman said.

"The First Minister does not take her instructions from Sinn Fein, but from the electorate."

Mr Bell has demanded a judge-led public inquiry.

He also accused DUP special advisers of blocking his efforts to clamp down on the excessively lucrative green heating subsidy late last year.

Also interviewed on BBC Northern Ireland's Nolan Show Investigation, Mrs Foster robustly rejected his assertions.

It made a remarkable televised bout of acrimonious claim and counter claim involving the leader of the region's largest party and one of her erstwhile ministerial colleagues.

The First Minister alleged Mr Bell was the one who acted aggressively and used his "physical bulk" to confront her.

Mrs Foster portrayed Mr Bell's interview as a "distraction" to mask his own failings in regard to the scheme, insisting that it was him who wanted to delay the shutdown of the massively overspent endeavour.

The scheme was developed during her time as economy minister.

On Thursday night she apologised for not implementing more controls at its inception, but claimed the Executive could take action that would potentially halve the overspend.

"Of course I'm sorry I didn't put in cost controls," she said.

Mrs Foster will face a motion of no confidence in the Northern Ireland Assembly on Monday when she makes a statement on the furore in a specially recalled sitting.

Mr McGuinness's call for her to stand aside has fuelled speculation about how his party will vote.

All eyes will also be on Mr Bell to see how he votes if he attends.

The DUP's electoral strength means the motion tabled by the SDLP will have no practical effect but it nevertheless will signify the strength of feeling from Sinn Fein and the Opposition.

The error-ridden RHI was designed to incentivise businesses to replace old heat sources with new eco-friendly alternatives, such as wood pellet boilers. But it ended up paying applicants more than the purchase price of the fuel.

There was no cap on the subsidy payments, so essentially the more heat you generated, the more public money you were paid. For every £1 of fuel bought by businesses, they got paid around £1.60 through the scheme.

The Assembly's Public Accounts Committee (PAC) is probing the affair but opposition parties have claimed its inquiry does not have the necessary degree of independence given a number of its members are DUP MLAs.

Mr Bell, who succeeded Mrs Foster as economy minister, claimed there had also been an attempt to remove references to her in documents related to the doomed scheme.

The DUP has rejected that allegation as a "hamfisted diversionary tactic" to mask his own "crucial and catastrophic" failure to take earlier action to clamp down on the overspend.