Northern Ireland's Deputy First Minister Martin McGuinness has called on First Minister Arlene Foster to "stand aside" to allow an investigation into a controversial green energy scheme that has left Stormont facing an estimated £400m overspend.

The error-ridden Renewable Heat Incentive (RHI) scheme 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.

There are claims a farmer in Northern Ireland is set to pocket around £1 million in the next 20 years for heating an empty shed.

Speaking this afternoon, Mr McGuinness said: "I spoke by phone this afternoon with the First Minister, Arlene Foster. 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."

"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.

In addition, I also said that, in the public interest, she should stand aside from the role as First Minister while that investigation is under way 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," said Mr McGuinness.

The intervention by the DUP's coalition partner in the Stormont Executive came after DUP leader and First Minister Arlene Foster said sorry for not implementing cost controls in the ill-fated RHI.

The "cash for ash" controversy escalated yesterday when former DUP economy minister Jonathan Bell broke ranks to level a series of explosive claims against Ms Foster and party advisers.

In an extraordinary TV interview that laid bare a bitter rift in a party known for its internal discipline, a tearful Mr Bell claimed a "highly agitated and angry" Ms Foster demanded he kept the RHI open for an extra fortnight, in the face of a Treasury warning, during a stormy showdown at Stormont when he was still economy minister earlier this year.

Mr Bell, who has demanded a judge-led public inquiry, also accused DUP special advisers of blocking his efforts to clamp down on the excessively lucrative green heating subsidy late last year.

Ms Foster was also interviewed on BBC Northern Ireland's Nolan Show Investigation and rejected his assertions in robust terms.

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

The first minister alleged that Mr Bell was the one who acted aggressively in the disputed meeting. The named special advisers also denied his claims of undue influence.

The DUP leader continues to face down calls to resign, or at least temporarily stand down pending investigation into her involvement in the RHI.

The scheme was developed during her time as economy minister.

Last night she apologised for not implementing more controls on the scheme 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.

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

The DUP's electoral strength means the motion tabled by the SDLP will have no practical effect but it nevertheless signifies the strength of feeling on the opposition benches, where there have already been vocal calls for a public inquiry.

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.

Conor Murphy, a Sinn Féin Assembly member and former Stormont minister, said: "There needs to be an independent investigation into this in which all information, including internal departmental papers, records, and email trails should be fully disclosed by all of those involved.

"We also need to know who benefited from this flawed scheme and who made the decisions surrounding its design, operation, and failure to monitor and slowest to close the scheme.

"In addition there needs to be urgent action to reduce the impact of this debacle on our public finances and on our public services.

"It is not acceptable that our health, education and other vital frontline services are suffering as a result of a scandal for which no-one has yet been held to account."

Speaking on RTÉ’s Six One, DUP MLA Christopher Stalford said as far as the DUP is concerned, Ms Foster will not be stepping aside to allow an investigation into the scheme.

He said the call from Mr McGuinness is misplaced.

"I believe she has behaved with honesty and integrity throughout this process, and I believe she should be allowed to continue with her job," Mr Stalford said.

Small street protests held urging Foster to quit

Street protests have been staged in Northern Ireland's two main cities calling on Ms Foster to resign.

The small scale demonstrations in Belfast and Derry were organised by left wing political party People Before Profit.

Between 200 to 300 people attended the event in Belfast, with fewer than 100 in Derry.