Mairtin O Muilleoir, who took over Stormont's finance ministry in May, is pictured on a TV screen in Stormont's Parliament Buildings.

He said up to £600 million could be “haemorrhaged” from the public purse – rather than just £400m-or-so as thought over 20 years.

Speaking on the radio Friday, Mr O Muilleoir said that the public purse is “haemmohaging” cash under the scheme, and that the overspending on the RHI scheme poses an “existential” threat to the Province’s budget.

As to its total cost, he said: “I know the figure of £400m has been used, but actually... over 20 years [its] the best part of £600m.”

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Mr O Muilleoir had also told the BBC’s Good Morning Ulster show: “There are hundreds of people abusing this scheme; there are hundreds of people using heat in an ineligible way.”

The number of RHI heating systems which have actually been inspected by someone to check if they are compliant is extremely low, and when he was asked how he could came up with his figure that “hundreds” of people are abusing the system, Mr O Muilleoir said he was basing his figure on the sheer amount of money which the government is paying out.

“If this was used in an eligible way, we would not be [subsidising] so much heat,” he said.

When it comes to those abusing the system, his finance department told the News Letter he was speaking specifically about non-domestic users of the RHI scheme, such as farms or companies.

This is an important distinction, because there are fewer non-domestic users of the RHI scheme (totalling about 2,100 applicants) than there are household ones (totalling about 2,700 applicants).

Yet despite being smaller in number, these non-domestic users receive a far greater share of public money; they are set to get an estimated sum of about £1.2bn over 20 years, compared to the £30m which the domestic users will get over the same period.

Despite Mr O Muilleoir’s figure of £600m, late on Friday the Department for the Economy – which is directly responsible for the scheme – cited the estimated cost to Northern Ireland as being about £490m over 20 years.