Further evidence linking Arlene Foster directly to the critical failure which left the Renewable Heat Incentive scheme open to gross abuse has emerged.

The First Minister, who today faces an Assembly motion which would exclude her from office if it is passed, has repeatedly argued that it was her officials who were to blame for the catastrophic decision to discard cost controls.

Those controls would have included a cap on payments, limited the cost to taxpayers, but Mrs Foster has said that no submission was made to her recommending that such controls be part of the scheme.

Opposition MLAs have argued that the minister must take responsibility for what went on during her tenure at the department.

Now an obscure legislative document which accompanied the initial non-domestic RHI regulations in 2012 has emerged.

That document – the Regulatory Impact Assessment, contained within the explanatory memorandum to the RHI statutory rule – shows that on 13 April 2012 Mrs Foster signed a declaration which said: “I have read the Regulatory Impact Assessment and I am satisfied that the benefits justify the costs”.

The document signed by the then minister also justified the creation of a separate Northern Ireland scheme rather than following out the GB scheme by saying that “tariffs developed specifically for the Northern Ireland heat market which will utilise available funding most efficiently”.

That statement was made despite the fact that cost controls had been removed from the scheme – something which meant that it was from the very start open to abuse and wildly spiralling financial commitments, as ultimately happened last year when 900 applications were received within a six-week period.

Last night the DUP said it would be “ludicrous” to blame any politician for the failure to retain the GB scheme’s cap on payments.

But TUV leader Jim Allister said that the ‘regulatory impact assessment’ to the regulations was highly significant in establishing the minister’s role in the process.

The North Antrim MLA said: “This is not signed by some hapless civil servant, but by Arlene Foster. So, there is no room for her to blame civil servants now.”

The document also made explicit that “the target must be delivered within the agreed budget of £25m to 2015 provided by Her Majesty’s Treasury”. And it said that only £25m would come directly from the Treasury, adding that “additional funding post-2015 will need to be negotiated with [Westminster departments] DECC and HMT in due course”.

The News Letter asked the DUP several questions about the document, including why Mrs Foster did not insist on the cost controls being included before signing the declaration that “the benefits justify the costs”.

In a detailed statement, the party responded last night: “Such questions are asked with the benefit of hindsight. At the time the scheme was being developed, there were external consultants engaged by the Department of Enterprise Trade and Investment as well as the department’s own specialist staff who worked on the proposals. “No recommendation ever came before the then minister to introduce tiering and throughout Arlene Foster’s period as Enterprise Minister there was an underspend in the scheme.

“When it was being launched, the chair of the Assembly’s Enterprise Committee stressed the degree of scrutiny which RHI had undergone. In October 2012 he told the Assembly: ‘The Committee scrutiny of the development of the renewable heat incentive has been considerable and reflects the importance and long-term nature of the proposals. Before supporting the RHI, the Committee sought and received assurances on incentive and tariff levels, banding levels, incentives for domestic customers, payments to participants and support levels for the renewable heat premium payment scheme.’”

The DUP statement added: “A key failing at the heart of this saga is detailed in the NI Audit Office report of July 2016. The Audit Office records that DETI incorrectly reported in the 2012 business case for RHI that there was no need to consider cost tiering controls ‘because the rate proposed was lower than the cost of fuel and therefore there would be no incentive to abuse the system by generating heat just to claim the subsidy’.

“This fundamental mistake informed the administration of the system for several years. It is ludicrous to blame any politician for this.”

In her interview with Stephen Nolan last week, Mrs Foster was challenged about the decision to take the cap out of the legislation.

She responded: “With the benefits of hindsight there should have been tariffs put in place but there weren’t at that time.

“I left the department in May of 2015. No problems were highlighted to me, my officials, there was no recommendation in relation to doing anything differently.”

Last night Mr Allister also drew attention to another portion of the ‘explanatory memorandum’ to the RHI regulations in which it stated clearly that the Treasury (HMT) “has advised that £25m of funding will be made available for a Northern Ireland RHI. This funding is spread over the spending period between 2011-2015... Additional funding post 2015 will need to be negotiated with DECC and HMT in due course.”

Mr Allister said: “Yet, before 2015 was out DETI had run up spending commitments of approximately £1 billion.

“Beyond 2015 any further funding required to be negotiated with Treasury.

This, along with the failure to get DFP re-approval by April 2015, guaranteed a scheme that had run out of control, with no safety net from the Treasury.”

This morning the Assembly will return for an emergency debate on a statement by Mrs Foster, then an SDLP motion (bvacked by all opposition parties) ttempting to exclude her from office for six months and then a Sinn Fein motion calling for Mrs Foster to temporarily stand aside and for an independent inquiry to be set up.

The SDLP motion will not pass unless there is a major rebellion in DUP ranks, because the Northern Ireland Act states that an exclusion motion must have the support of a majority of both unionist and nationalist MLAs to pass. The DUP has 37 (not counting the Speaker) of the 54 unionist MLAs.

Yesterday the SDLP claimed that “pride” had prevented the First Minister from standing aside “to facilitate a full inquiry into the biggest public finance scandal in the history of devolution” and that similarly “pride prevents Sinn Féin from signing up to a cross party proposal to stand the First Minister aside to facilitate a full inquiry into the biggest public finance scandal in the history of devolution”.

The spokesman added: “The public has had enough of politicians putting their own pride first and the public interest second. This is a time to restore faith in our institutions.”

Meanwhile, it has been claimed that Mr Bell is about to embark on a defamation action against Mrs Foster over allegations which she made in Thursday night’s dramatic Stephen Nolan interview. Mr Bell has not confirmed that he is to do so, having just said that he has instructed specialist libel solicitor Paul Tweed, but the Belfast Telegraph reported that Mr Bell has “robustly denied the First Minister’s claims and has told friends that he is determined to pursue the matter in the courts”.