Máirtín Ó Muilleoir made the comments in a private email which the RHI Inquiry has compelled his party colleague Conor Murphy – to whom it was copied – to release.

Within months of endorsing a strategy which added to the RHI bill, Mr Ó Muilleoir would be Sinn Fein finance minister, with responsibility of attempting to plug holes in Stormont’s budget created as a result of the huge RHI bill.

The revelation further undermines Sinn Fein vice president Michelle O’Neill’s claim at the height of the scandal in January 2017 that her party “shut it down straight away” when the problems became clear.

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The comments by MÃ¡irtÃ­n Ã“ Muilleoir (right) were made in a private email that was copied to SF colleague Conor Murphy

The email also backs up the evidence which former DUP Spad Tim Cairns gave to the inquiry earlier this month, when he said that Arlene Foster told him and his minister, Jonathan Bell, that it was Sinn Fein which wanted to keep the scheme open for two more weeks.

By that stage, the scheme was operating with cost controls – but keeping it open added to the bill to taxpayers.

Mr Ó Muilleoir had been emailed by Aisling Brady, community development manager at the Torrent Complex in Donnaghmore, on February 11 2016, the day that Mr Bell announced that RHI would be kept open for another two weeks until February 29.

However, she told the Sinn Fein MLA that “still isn’t enough time for us”.

She said:” I hope you can persuade Mr Bell to understand the impossible position he is putting people in who are in the system and were aiming for the 31st March deadline.”

She signed off her email by saying: “Thank you again for your campaign against this unjust decision.”

Mr Ó Muilleoir replied within an hour, saying: “I’m very disappointed for all affected though am pleased the minister did not go ahead with plan to close scheme next week as he proposed until we protested in committee and at question time. We are assessing our options at the minute and will do our utmost to have as many projects as possible green-lighted.”

Internal civil service emails show that there was high level concern about the two-week delay, because of the fact that by then Stormont knew that it - and not the Treasury, as it had initially believed - was footing the bill for the vast RHI overspend.

Eugene Rooney, a deputy secretary in the Department for the Economy, emailed his permanent secretary, Andrew McCormick, to relay that a senior official in the Department of Finance “has raised whether this announcement is consistent with what the Executive has agreed - he feels it may not be...[he]is also concerned that there will be an additional budget implication...and do we have an estimate?”

Dr McCormick replied to say that the delay had been agreed “through the usual channels” and said “my understanding is that legitimate authority and approval exists for the announcement”.