Stormont's under pressure First Minister is to address a special sitting of the Northern Ireland Assembly as calls for her to resign over her handling of a botched green energy scheme intensify.

Democratic Unionist leader Arlene Foster will make a "full statement" to MLAs on Monday as the devolved institutions are recalled to address the growing political crisis around the Renewable Heat Incentive.

Errors in the now closed scheme, which was developed under Ms Foster's watch when she was Stormont's economy minister, have left the powersharing administration in Belfast facing an estimated £400 million overspend.

An initiative that was supposed to help businesses mitigate the costs of running eco-friendly boilers actually ended up paying out more than the cost of the fuel - so the more people burned, the more public money they earned.

There are claims some applicants to the scheme are in line to pocket £1 million over 20 years for heating empty sheds.

Ms Foster and party colleagues have insisted that steps will be taken in the New Year that will reduce the impact on the public purse.

However critics have questioned what action can be taken, given Stormont has entered into legally binding long term contracts with users of the RHI.

Ms Foster has so far faced down calls to step down, insisting that she acted correctly and mistakes were made by officials.

On Wednesday, the DUP was forced to deny media claims that advisers within the party and Ms Foster's office tried to delay the scheme's closure.

Later, ministers in the DUP and Sinn Féin-led executive met at Stormont Castle to discuss the furore.

Afterwards, they announced plans for the special sitting. Assembly speaker Robin Newton subsequently rubber stamped the request.

Earlier this week Ms Foster said she would not be stepping down from her role as First Minister over the controversial scheme.

In a statement today, Ms Foster and Sinn Féin Deputy First Minister Martin McGuinness outlined the purpose of the sitting.

"This will facilitate a full statement to be made by the First Minister to members on the matters of public concern relating to RHI," they said.

"RHI was discussed by the Executive today and ministers around the table underlined the seriousness of the issues involved and the importance of restoring public confidence.

"It was also emphasised that detailed plans are being finalised to significantly reduce the projected losses in the years ahead."

Emerging from the executive meeting, Sinn Féin Finance Minister Máirtín Ó Muilleoir said he was confident the costs could be reined in.

"One thing is for sure - we need to take action," he said.

"The public is saying, and I agree with them, that the scheme was introduced in a wrong way, it was mess.

"There is no arguing that it was badly handled and I think it's not good enough to say to the public that we can't do anything."

The First Minister has been accused of not doing enough to pursue whistleblower allegations that sought to expose flaws in the system.

Her acknowledgement that one of her former special advisers - Stephen Brimstone - was an RHI applicant, and confirmation from the DUP that the brother of Andrew Crawford, another of her former special advisers, was also on the scheme have led political rivals to assert that Ms Foster must have had an intimate knowledge of its workings.

There is no suggestion Mr Brimstone or Mr Crawford's brother were anything other than legitimate claimants.

The RHI aimed to cut the cost of green energy to encourage people to move off fossil fuels but ended up landing ministers with a massive overspend.

It incentivised the installation of costly eco-friendly heating systems by paying a tariff per kilowatt of heat burned over a 20-year period.

However, unlike in the rest of the UK, in Northern Ireland no cap or payment tier system was placed on the money that could be claimed in proportion to the size of boiler and the hours it was operated.

That resulted in the RHI tariff paid out being higher than the cost of fuel needed to run the boilers.

Thousands signed up to the RHI - a deluge that ultimately forced its closure, but not before Stormont had been left with a huge future bill.

Overall, more than £1bn of public money will be paid by 2036 to Northern Ireland-based businesses which signed up to the scheme. Around £400m of that will be paid out by the Stormont Executive.

DUP releases email to disprove 'myth'

The DUP later released the copy of an email which it claimed "nailed the myth" that Ms Foster had not done enough to pursue whistleblower claims.

The party said the correspondence was sent by a whistleblower but that it made no mention of RHI concerns. It said those concerns were only aired with officials when Ms Foster referred the email to them.

DUP deputy leader Nigel Dodds called on opposition parties and sections of the media to "retract allegations" against the First Minister.

"There has been a quite scurrilous attempt to blame Arlene Foster for the RHI saga," he said.

"The endlessly repeated claim has been that she failed to follow up on the whistleblower concerns.

"Now we know that no such concerns were raised with her. This is no criticism of the lady in question. She has behaved with great integrity throughout and even told the media that her initial email to the minister did not go into detail.

"The Department has admitted it should have responded differently when the RHI (concerns) were subsequently aired.

"The Department's permanent secretary has made clear throughout that Arlene Foster acted entirely appropriately.

"Sadly Opposition parties and some section of the media didn't want to know that. They have been engaged in a disgraceful and baseless blame game for which they should now apologise."

Mr Dodds added: "No one is denying for a second that RHI is a totally unacceptable state of affairs. But the attempts to blame Arlene Foster have descended to hysterical levels."

SDLP to table motion of no confidence

SDLP leader Colum Eastwood said he would table a motion of no confidence to exclude Ms Foster from office while the RHI is being investigated.

The move is unlikely to succeed in the face of DUP opposition.

"In light of the First Minister's continued failure to answer the most serious questions about the biggest public finance scandal in the history of devolution, it's clear that the matter has to be forced," Mr Eastwood east.

"The SDLP will table a motion of no confidence to exclude Arlene Foster as First Minister while she continues to face these critical questions and until she has accounted for her conduct in relation to the Renewable Heat Incentive fiasco."

He added: "She has lost all credibility and anything less will further erode faith in our institutions. If the First Minister will not stand aside, then the Assembly must act to remove her from office and fully scrutinise this scandal."

After the executive meeting and in a subsequent radio interview, Mr Ó Muilleoir repeatedly declined to state whether he retained confidence in Mrs Foster.