The report from DOJ Inspector General Michael Horowitz will be reportedly 500 pages long and is scheduled to be delivered to Congress at noon, with a public release three hours later.
He concluded, according to a copy of the report obtained by Bloomberg News, that Mr Comey damaged the law enforcement agencies' image of impartiality, even though he was not motivated by political bias.
Former FBI Director Jim Comey violated Bureau procedures and "negatively impacted the perception of the FBI" in his handling of the Hillary Clinton email probe, the Bureau's Inspector General's report states. Former Attorney General Loretta Lynch, former FBI Director James Comey, and ex-deputy director Andrew McCabe will all be given a thorough review for their conduct during the 2016 election.
Trump's supporters have eagerly awaited the report in hopes that it would skewer the judgment of James Comey, who was sacked as Federal Bureau of Investigation director past year.
"The stark conclusion we draw after reviewing this report is that the FBI's actions helped Donald Trump become President", said Reps.
The president "is really anxious to see the report", said a person familiar with the planned Thursday briefing.
Again, Clinton aides, Democrats and former Justice Department officials from both parties criticized Comey, saying he should not have publicly discussed an investigative action especially before he knew whether the emails were significant.
Also investigated by the IG was Comey's decision, against the recommendation of the Justice Department, to reveal to Congress that the FBI was reopening the email investigation following the discovery of additional messages.
Trump is widely expected to use the inspector general findings to launch fresh attacks against not only Clinton but also the law enforcement officials behind special counsel Robert S. Mueller III's Russian Federation probe, which Trump has repeatedly referred to as a "witch hunt".
Clinton has since said that Comey's letter contributed to her unexpected defeat by Trump.
"If anyone else shows up in this report to have done something that requires termination we will do so", he concluded.
Comey already has been criticized for waiting a month to act during the 2016 presidential election campaign after thousands of emails were discovered on Weiner's laptop that appeared to be relevant to the Clinton investigation.
The report's findings are set to be made public later Thursday. Comey informed Congress of the emails October 28, 2016.
On Wednesday, Attorney General Jeff Sessions said the report could prompt more firings.