Arlene Foster wrote the letter in 2013

Mrs Foster has consistently said that uptake of the RHI scheme was initially low and that this was one of the reasons why her officials were more focussed on publicising it than introducing cost controls.

In December Mrs Foster told the Assembly that “scheme uptake was initially low in the first few years with only 409 applications received by the end of 2014, leading to an underspend of around £15 million during the first four years”.

Former DETI permanent secretary David Sterling last year told MLAs that when he left DETI in July 2014 “there had been around 216 applications to the scheme...there had been a low uptake.”

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His successor, Andrew McCormick, told the Public Accounts Committee (PAC): “The context is that the uptake was slow. The initial uptake of the scheme was relatively low, so people were not so concerned about it.”

And, referring to the point where he took over in July 2014, Dr McCormick said: “Throughout that period, uptake was low, and there would not have been an alarm bell ringing.”

He went on to say: “The key point that stands here is that uptake was low, so nobody was getting excited.”

However, now a letter which Mrs Foster wrote to her then Westminster counterpart, Greg Barker, in December 2013 – just a year into the scheme – has emerged.

The letter was released to UUP peer Lord Empey under the Freedom of Information Act.

In the letter, Mrs Foster said: “The Northern Ireland RHI has only been in operation for 12 months, yet there has been an encouraging level of uptake, with the number of applications for the Northern Ireland scheme being around 7% of the number received by the GB scheme during its first year of operation.”

The 7% quoted is far above Northern Ireland’s 2.8% of the UK population.

The DUP did not respond to questions about Mrs Foster’s letter.

The Department for the Economy said: “The low uptake for the Scheme was highlighted in the [Audit Office] report which noted ‘..a considerable under spend totalling around £15 million up to 2014-15’. The [Audit Office] report also noted that ‘as a result of the low uptake, a lot of the department’s focus at this time was on identifying ways to increase demand”.

“The references by officials at PAC evidence sessions to low uptake at the beginning of the non-domestic RHI scheme were in that context.

“Eighty five applications had been made up to the end of December 2013; 409 by December 2014; 1,805 by December 2015; and 2128 at the end of February 2016 when the Scheme was suspended.