MÃ¡irtÃ­n O Muilleoir didn't put any cost controls on the RHI inquiry. Photograph by Presseye/Cameron Hamilton

Stressing that it would be a matter for the judge, the minister told the Assembly: “If pressed, I would think it appropriate for us to have a report six months after the inquiry starts, but that, in my view, is a matter for the chair.”

However, it is now clear that Mr Ó Muilleoir’s estimate was wildly ambitious and the inquiry is likely to take more than a year to complete its work.

On Thursday, inquiry chairman Sir Patrick Coghlin resolutely refused to even give an estimate for when it will be complete.

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However, he said that inquiry evidence hearings will not even begin until “early autumn” – about eight months after Mr Ó Muilleoir made his announcement.

Sir Patrick also stressed that his inquiry is not time-bound.

In fact, it has been confirmed to the News Letter that there is no control over the inquiry either in terms of time or costs.

Mr Ó Muilleoir effectively gave it a blank cheque on both fronts, in an effort to stress just how independent of Stormont it would be – despite the fact that the inquiry is investigating a scheme whose central failure was an absence of any cost controls.

Sir Patrick said that he was “acutely aware” that the inquiry “has a public duty to keep the time and cost of any required alterations within reason”.

In January, Mr Ó Muilleoir effectively told the Assembly that he had advised the judge to be cautious on costs but that there was no statutory limit.

He told MLAs: “Sir Patrick Coghlin understands it is the wish of all of us that the costs of the inquiry do not add to public disquiet over RHI. He is cognisant that the terms of reference encourage him to be cautious about costs.”

Already, an indicative budget of more than £4 million has been set aside for the inquiry for the next year.

One area in which the inquiry will save public money is by holding its hearings in Stormont’s Senate Chamber – although structural work is under way there to make it suitable for the inquiry – rather than renting a venue elsewhere.

On Thursday, Sir Patrick delivered a robust message about his determination that the inquiry will be both rigorous and utterly independent.

The former judge said he wanted to reassure the Northern Ireland public that the inquiry “will fearlessly examine the evidence relating to the RHI scheme; and that it will do so without interference from any quarter in order to establish the facts about the scheme and those connected to it”.

Sir Patrick also clarified that he has designated three bodies as “core participants” – the Department for the Economy, the Department for Finance and Ofgem.