A former DUP special adviser who was heavily involved in Northern Ireland’s botched green energy scheme has said he felt “expendable”.

Dr Andrew Crawford worked with party leader Arlene Foster when she was a minister in the Stormont Assembly.

He said he never tried to postpone cost controls which civil servants overseeing the scheme wanted to introduce in October 2015 as the Renewable Heat Incentive (RHI) spiralled out of control.

In written evidence to the RHI public inquiry he wrote: “It may have been the case that I was viewed as expendable whereas Timothy Johnston was not.”

Mr Johnston is the party’s chief executive and was also a DUP special adviser.

The chairman of the Renewable Heat Incentive inquiry Sir Patrick Coghlin (RHI Inquiry/PA)

Senior civil servant Andrew McCormick has told the inquiry the DUP was concerned to “deflect” reference to Mr Johnston and agreed to name Dr Crawford as instigator of the delay.

Dr Crawford said neither he nor Mr Johnston had attempted to slow the passage of cost controls.

Tim Cairns, another former special adviser who worked with former Stormont enterprise minister Jonathan Bell, disagreed.

The inquiry chairman, retired judge Sir Patrick Coghlin, said: “One of you is accurate or telling the truth about that – not both.”

He said he was trying to assess their credibility, adding: “Both of those cannot be true.”

Earlier stages of the inquiry have heard there was a mistaken belief among those involved in designing the RHI that the Treasury would pick up the bill for overspending on subsidies.

The intention was to have as many as possible enrolled to attract funds to Northern Ireland.

Mr Cairns said he believed the bill for RHI was being covered by Westminster and it was a “valid” step to delay change beyond the October 1 date favoured by civil servants.

He said: “It’s better to be spent in Belfast than Bristol.”

The scheme became over-subscribed by wood fuel burner owners and threatened to incur a massive overspend on public coffers.

Concerns centred on how long it took to close after civil servants became aware of the flaws.

The devolved powersharing institutions collapsed early last year after Sinn Fein walked out over the DUP’s handling of the RHI, and repeated rounds of negotiations have failed to resurrect them.

Dr Crawford said: “I believe Timothy Cairns was concerned about further negative publicity following the amendment to the NIRO (energy micro-generation) scheme, and in the context of the poor relationship between Jonathan Bell and Timothy Cairns, he was concerned that he could lose his position as special adviser if he wasn’t careful in how he handled these changes.”

Mr Cairns said they should all have sat around the table and thrashed out the issues.

He said: “I was relying on phone calls and this shuttle diplomacy that really was deficient.

“You have to be able to say that you did your best or, electorally, that is going to be a problem.”