The DUP "camouflaged" how it appointed special advisers, a public inquiry has heard.
Former Stormont minister Jonathan Bell has claimed that he was instructed who to appoint as his special adviser in May 2015.
A code of conduct says that ministerial advisers should be appointed by ministers.
It also says that when selecting a special adviser, ministers should consider a number of candidates.
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Mr Bell said that previously DUP ministers had had a "free choice" to appoint who they wanted to be their special adviser.
But he said that changed following an incident where a minister had appointed a special adviser the party did not like.
In May 2015, he said he was ordered by the DUP to appoint Timothy Cairns as his special adviser.
He signed a "pre-written" letter saying he had considered a number of candidates and found Mr Cairns to be the most suitable.
"I have concluded that Timothy Cairns is by far the most qualified candidate available," wrote Mr Bell, and he went on to describe Mr Cairns as "highly capable".
Mr Bell said he thought the process was lawful at that time.
"It's clear from what you are showing me that that was a contravention with the legal position but I thought it was lawful at the time," he said.
But Mr Bell told an inquiry into how costs for a renewable energy scheme in Northern Ireland spiralled, that he was told by the DUP to appoint Mr Cairns.
He told the inquiry he was "content" with Mr Cairns as his special adviser, but would have preferred to have had a choice.
Inquiry chairman, the Rt Hon Sir Patrick Coghlin, appeared taken aback at the evidence.
He said: "You were signing a letter that was consistent with the code, but that was not what this party had decided?"
Sir Patrick went on to say there is a "very real concern" that the appointment process for advisers was being "camouflaged".
Mr Bell responded: "That's exactly right."
Mr Bell became minister of the Department of Enterprise, Trade and Investment, which ran the RHI scheme, in May 2015.
The RHI scheme was intended to encourage businesses to switch from fossil fuels to green alternatives like biomass.
By the summer of 2015 the cost to the taxpayer was beginning to spiral as firms realised how lucrative the incentives on offer were.
Mr Bell has insisted to the RHI inquiry that he was not made aware of budget concerns around the scheme until "some time around the summer".
He did not recall the exact date he became aware of it.
"I honestly can't precisely recall, it was not a formal meeting, it was a conversation," he told the inquiry, adding it had been the department's permanent secretary Andrew McCormick who first raised it with him.
Mr Bell has claimed the item was on the agenda at a top level department meeting on June 8, but that his special adviser Timothy Cairns insisted it was not discussed that day.
"My special adviser said 'oh we are not dealing with this today ... I am discussing that with the other spads (special advisers)'," he said.
"There was no further discussion on that."
Mr Bell added that with hindsight, the situation where a minster was not given information was "quite outrageous".
"None of that information, quite outrageously now, was given to the minister," he told the inquiry.
Mr Bell also claimed that had he known about the budget concerns around the scheme, he would have closed it to new entrants as early as May 2015.
