Witness statements published by the RHI Inquiry last night ahead of Mr Ó Muilleoir’s appearance today before Sir Patrick Coghlin’s public inquiry show that he explicitly requested the agreement of the close confidante of Gerry Adams before signing the document.

The revelation adds weight to the belief of the DUP and others that Sinn Fein’s Stormont operation was so ultra-centralised that even Stormont ministers were operating in an environment in which they were being effectively overseen by unseen republicans.

A witness statement from former DUP minister Simon Hamilton refers to an email from Mr Ó Muilleoir to Ted Howell, a long-standing confidante of Mr Adams, which the inquiry has obtained but has not yet published in full.

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Sinn Fein MLA and Finance Minister Mairtin O Muilleoir speaks to the media after meeting DUP Economy Minister Simon Hamilton for a 'Clear the Air meeting last year over the controversial RHI Renewable Heating initiative. Photo Colm Lenaghan/Pacemaker Press

The email was in January 2017, at the height of the RHI scandal, and concerned a business case from Mr Hamilton which requested the finance minister’s approval for legislation which retrospectively slashed RHI subsidies, cutting the total cost to taxpayers.

Mr Hamilton said that it was “disturbing” for him to learn of the email’s contents, which he said showed that “the business case did not appear to be being considered on its merits” but was being considered by Connolly House (Sinn Fein headquarters).

Mr Hamilton, the then economy minister, said that in the email to Ted Howell Mr Ó Muilleoir asked: “Would you [Ted Howell] be content if I [Mr Ó Muilleoir] were to sign off the business plan on Wednesday?”.

Mr Hamilton said that was “over a week after non-partisan DOF officials had recommended approval of the business case”.

It is unclear why the then finance minister was consulting Mr Howell about such a massive financial decision. The inquiry published a written statement from Mr Ó Muilleoir last night but in it he was not asked about the correspondence.

During the Troubles Mr Howell was Sinn Fein’s director of foreign affairs and described by the veteran journalist and author Ed Moloney as “a reticent but enormously influential activist who doubled as chairman of the party’s Think Tank, the body that directed the peace process for the SF leadership”.

Mr Moloney, author of A Secret History of the IRA, described Mr Howell as “Adams’ most trusted advisor and counsellor” but also “a highly secretive figure” who was “arguably one of the most influential figures in the Provisionals”.

Another veteran security journalist, Brian Rowan, described Mr Howell as having been “part of Gerry Adams’ ‘kitchen cabinet’ for decades”.

A Washington Post article earlier this year revealed how Mr Howell acted as a go-between on behalf of the IRA when the terror group was beginning to make connections with senior US figures in the early 1990s.

Mr Hamilton said that the “decision to grant or withhold approval for the 2017 regulations business case appears to have been taken more in Connolly House rather than Clare House [the Department of Finance headquarters]”.

He added: “There was no good reason to delay approval of the business case for the 2017 regulations and this foot dragging by Sinn Fein cast doubt on the efficacy of those regulations.”

The Sinn Fein minister received the business case on January 13 but did not approve it until January 25.

Mr Ó Muilleoir said that what he described as Mr Hamilton’s uncooperative approach “hindered the development of a solution to RHI”.

But Mr Ó Muilleoir accused his former DUP Executive colleague of not having acted with sufficient urgency prior to that point to bring forward credible proposals to cut costs.

He said that he did not believe Mr Hamilton’s legislation was the best way to deal with the overspend but that he gave approval “because my officials were willing to approve the business case and it was better than doing nothing”.

Among several pages of written questions to Mr Ó Muilleoir, the inquiry asked the businessman and politician why as minister he had used the email address mairtin@newbelfast.com rather than his official departmental email, which is held on departmental computers.

Mr Ó Muilleoir said that was the email he “normally used to communicate with party colleagues and constituents”. He said that “in terms of security I was and am confident the email account, the only email attached to a secure and stand-alone domain, is best-in-class”.

He also told the inquiry that when he sympathetically told a prospective RHI claimant that “we are assessing our options at the minute” to get as many as many people like her as possible on the RHI scheme he actually had no options in mind.

Mr Ó Muilleoir was also asked about the revelation that he privately took credit for keeping the RHI scheme open in February 2016 – and told a woman hoping to install a boiler that the party wanted to see as many people in her position getting approval as possible.

The Sinn Fein MLA told the inquiry that at that point he had been contacted by members of the public who had bought boilers “in the expectation of being accredited on the scheme” and “they informed me that abrupt closure would cause financial losses which could result in their organisations being forced to close”.

Mr Ó Muilleoir had referred in the email to “we”. However, when asked who the “we” referred to, the MLA said: “I was speaking only for myself”.