The Renewable Heat Incentive Inquiry report, which will be published on Friday, has cost the taxpayer an estimated £7m.
Sir Patrick Coghlin's report into the cash-for-ash scandal, which brought down the last power-sharing Executive, will be released at 2pm. The retired judge will then make a statement at Stormont.
Senior officials in the Department of Finance received the report on Thursday, but sources said that Sinn Fein Finance Minister Conor Murphy did not. There is speculation that it will be heavily critical of the Civil Service.
Key witnesses at the inquiry, including DUP leader Arlene Foster, have already seen the parts of the report referring to them.
The demeanour of senior DUP figures suggests they believe it contains nothing which will endanger Mrs Foster's leadership. She is expected to publicly respond to the findings this afternoon.
Lawyers for key witnesses will receive the full report on Friday morning. It is understood to run to over 550 pages.
Speaking ahead of its publication, UUP leader Steve Aiken said: "Despite the current challenges of Covid-19, the RHI report potentially represents the most significant critique of the last 10 years of DUP/Sinn Fein misrule.
"That a devolved democracy has been undermined by unacceptable political practices, coupled by a Civil Service that forgot its first role of 'speaking truth to power', is a sad indictment, both of our political system and, more widely, of how Northern Ireland is run."
Mr Aiken added: "Hopefully, the report will lead to a fundamental change in the culture of government. However, we have seen little willingness so far that the main protagonists are prepared to embrace badly needed reform."
Meanwhile, a poultry farming couple, who asked not to be named, have criticised the DUP's handling of RHI.
They said they had joined the scheme in 2015 in good faith and had benefited from the improvement in their chicken houses.
"We have, however, been financially crippled by the decreases in tariffs and are deeply saddened by the lack of integrity of our government, who made binding promises which they have now broken," they said.
"Over the past few weeks we have emailed Mrs Foster several times as we see her as our guarantor with the bank.
"This is because she signed the letter which our bank took as sufficient security to give us the necessary loan. We can no longer make repayments and were seeking guidance from her. Mrs Foster said that she could not comment until after the publication of the inquiry report."
The couple said that Mrs Foster had directed them to the Department for the Economy. But they said that Minister Diane Dodds had told them it was "too early to meet with folk such as ourselves".
The DUP has been asked for comment.
The RHI Inquiry heard oral evidence between November 2017 and April 2018. A total of 63 witnesses appeared before it. The transcripts of their evidence amounted to 17,054 pages.
A further 459 witness statements were received, while 815 section 1 notices compelled recipients to produce evidence.
A total of 1.3 million pages of evidence was submitted to the inquiry, plus 11,000 spreadsheets.
Sixty-nine lawyers were involved in the hearings, and 54 staff were employed by the inquiry.