A toxic relationship between former Enterprise Minister Jonathan Bell and his special adviser Timothy Cairns led to an atmosphere of distrust at the height of the Renewable Heat Incentive (RHI) crisis, the inquiry has heard.
The former Permanent Secretary at the Department of Enterprise, Trade and Investment (Deti) Dr Andrew McCormick was questioned about a breakfast meeting argument between the two men in London in June 2015.
- Two relatives of Arlene Foster's advisor had applied for the scheme
- 'Now you're going to listen to me' - how Bell says adviser spoke to him
The Deti Minister, his adviser and Dr McCormick had arrived in the city for a scheduled meeting with then UK Energy Minister Amber Rudd.
The inquiry heard the row concerned Mr Cairns questioning Mr Bell's ability to take decisions on his own without consulting others in the DUP and ended with Mr Cairns being told that he wouldn't be attending the meeting with Ms Rudd.
There was a "pretty sharp" exchange between the pair, according to Dr McCormick, but he added he did not see anything from either man that was threatening or intimidating.
But Dr McCormick said he sensed a "breakdown in trust" afterwards and that had "made the difficult RHI issue significantly harder to resolve".
He told the inquiry: "Tim Cairns' intervention was a sharper rebuke to a minister from a special adviser than I'd seen before. The tone and the language was terse and pointed and right at the borderline of normal conversation.
"It wasn't long before we realised this was not for us (Civil Service) to be involved in. It got sharp, so we got off-side."
Dr McCormick added he had not seen anything from either man that would have triggered a duty of care from him.
Following the meeting, Mr Bell said Mr Cairns was dismissed for the day, while Mr Cairns thought he was dismissed permanently.
Dr McCormick said: "I walked up to Whitehall place and Jonathan Bell came into the reception area and said briefly 'Timothy Cairns isn't coming to this meeting'. I heard more details later that day and in the following days.
"I heard that Tim Cairns had a medical certificate in relation to his absence after that. I didn't hear the word sacked or dismissed in relation to this issue, until long after the event. Not that long later he came back and we resumed working together, but I was aware there was a real difficulty in the relationship."
Mr Cairns previously recalled in his evidence to the inquiry that he had gone to the Deti permanent secretary with concerns about Mr Bell's behaviour, but that Dr McCormick told him that he could only act in relation to civil servants and there was nothing he could do.
Asked if he remembered that happening, Dr McCormick said he'd been "scratching my head" about whether it did.
"I think I would remember Timothy coming saying: 'You need to help me with a bullying problem.' I don't recall that," said Dr McCormick.
"The understanding was that the dispute between Mr Bell and his adviser was a matter for the DUP rather than the Civil Service."
During yesterday's hearing, Dr McCormick also told the inquiry he still struggles with why a whistleblower warning about the key flaw in the scheme was ignored. He said he only found out about businesswoman Janette O'Hagan's concerns after the RHI was closed.
Ms O'Hagan told the department in 2015 that the scheme was being abused and businesses were burning fuel to take advantage of the over-generous state subsidies on offer, but the warning was not followed up.
Dr McCormick said he could not understand why she had been ignored at a time when officials were beginning to realise they had a budget problem. He said he could see no "defensible explanation" why the information had not been acted on.
The hearing also heard evidence about the working culture within the enterprise department when problems with the scheme first began to emerge.
Inquiry panellist Dr Keith MacLean asked if staff were reluctant to speak out because of power struggles, or took the view that "if I do say something I'll get my head bitten off for it".
Dr McCormick admitted there should have been a message conveying confidence to staff.
The former senior civil servant also spoke of communication difficulties between the energy and finance teams when it came to looking at the scheme's budget, which led to a reliance on emails.
Dr MacLean asked: "Is this an example where someone should have gone along the corridor and banged heads together until they got an answer?"
Dr McCormick admitted there had been a culture of self-protection within the department.
Inquiry chairman Sir Patrick Coughlin asked why Dr McCormick did not ask more questions in May 2015 when the cost of the scheme had risen from £12m to £23m.
Dr McCormick conceded: "That should have happened. I agree. I didn't ask hard enough questions at the time. I deeply regret not asking more fundamental questions at that time".
Dr McCormick also told the inquiry that he did not recall any attempts from DUP spads to stop discussion of the RHI scheme in meetings during summer 2015 shortly after Jonathan Bell became Deti minister.
Mr Bell's claim that DUP advisers did not want the scheme to be discussed during ministerial meetings was also put to Dr McCormick.
Dr McCormick replied: "I don't see why anyone would withhold that fundamental information."
Mr Bell is to appear before the inquiry over the next two days.