WASHINGTON — Attorney General William P. Barr recently approved making public new details about a former F.B.I. informant at the heart of conservatives’ allegations about the Russia investigation, deciding to release information that had been blacked out in a highly anticipated inspector general’s report due out on Monday.

A representative from the office of the Justice Department inspector general, Michael E. Horowitz, told the former F.B.I. informant, Christopher Steele, on Sunday that the Justice Department had decided to allow for the release of the information, two people briefed on the situation said late on Sunday.

Mr. Steele was given no details about the information itself, nor was he told how it would affect the report’s portrayal of him, the people said. Mr. Horowitz is expected to be critical of Mr. Steele, a British former spy who compiled a dossier of salacious, unverified information about President Trump. F.B.I. officials relied to some degree on the dossier to seek a court order for a wiretap of a former Trump campaign adviser, and the president’s allies have seized on the issue to make broad claims about the sprawling Russia investigation.

The notice to Mr. Steele on the eve of the report’s release was highly unusual. Like the other witnesses interviewed for the inspector general’s report, Mr. Steele had earlier reviewed the findings that are pertinent to him, and he was given a chance to comment on them. In this case, Mr. Horowitz’s office did not detail for him the additional information and gave him no opportunity to respond for the report to be released on Monday.

Spokeswomen for Mr. Barr and Mr. Horowitz declined to comment. Mr. Steele did not respond to a request for comment.

Mr. Barr had pushed the F.B.I. in recent months to declassify the entire forthcoming report, arguing that it would be better for the findings to be as transparent as possible. That process has taken months and only recently wrapped up.

It was not clear why the information about Mr. Steele had originally been blacked out in Mr. Horowitz’s report. Though Mr. Horowitz’s office operates independently, the attorney general and other senior law enforcement officials routinely review its reports to black out sensitive details like information that is classified or involves a continuing investigation.

During his investigation, Mr. Horowitz asked witnesses about an assessment of Mr. Steele by MI6, the British spy agency, which it provided to the F.B.I. after the bureau had received the Steele dossier. MI6 officials described Mr. Steele as honest but said that he sometimes pursued targets that others believed were a waste of time, two people familiar with the assessment have said.

Mr. Steele’s dossier claimed that Russian intelligence officers had used blackmail and bribery to try to turn Mr. Trump into a source, and Mr. Horowitz’s investigators examined whether law enforcement officials improperly relied on it to seek a wiretap for the former Trump campaign adviser, Carter Page, who visited Russia during the campaign.

Mr. Steele has made clear to associates that he considered the dossier to be a starting point for further investigation, not established facts. It has grown clear in the years since Mr. Steele compiled his dossier that while many Trump aides welcomed contacts with Russians, some of its most sensational claims appeared to be false or were impossible to prove.

The more than 400-page inspector general’s report is expected to debunk the idea that the F.B.I. relied on the Steele dossier to open its Russia investigation, though it was used to apply for the warrant to wiretap Mr. Page. It is not clear the extent to which the application for the warrant relied on the dossier.

Mr. Trump’s allies have emphasized that Mr. Steele compiled the information for a research firm, and that the firm was paid by lawyers who worked for Hillary Clinton’s campaign and the Democratic National Committee. They have argued that it was inappropriate to use partisan opposition research in a sensitive application to wiretap a person associated with a presidential campaign.

The inspector general is expected to say that the Justice Department did not know the identity of Mr. Steele’s patrons when they used some of his information in the original October 2016 application to wiretap Mr. Page. But he will be critical about the fact that law enforcement officials did not change the language about who paid for the research in later wiretap renewal applications, after officials learned that Democrats had funded the research, according to people familiar with a draft of the report.

Mr. Horowitz will also fault the F.B.I. for not telling the judges who approved the wiretap applications about potential flaws with the dossier.

He is also expected to criticize contact that Mr. Steele had with Bruce G. Ohr, a Justice Department official and Russian organized crime expert who shared information from Mr. Steele with the F.B.I. even after the bureau ended its relationship with him as an informant in the fall of 2016.

Mr. Ohr kept those meetings with Mr. Steele from his superiors, which Mr. Horowitz is expected to criticize. Mr. Ohr’s wife also worked for the research firm that had hired Mr. Steele, Fusion GPS.

Matthew Rosenberg contributed reporting.