The source, who is not identified, “raised doubts about the reliability of Mr. Steele’s descriptions of information in his election reports,” saying Mr. Steele had “misstated or exaggerated” what he had been told, the report said. Though Mr. Steele’s report on Mr. Trump’s supposed 2013 encounter with Russian prostitutes said it had been “confirmed” by a hotel staff member, the source told the F.B.I. agents that the story of the purported episode at Moscow’s Ritz Carlton was just “rumor and speculation.”

Mr. Steele told the inspector general’s office that while he debated whether to include the salacious sexual allegations, he decided that “we have to be faithful to all of the information the source provided” and not avoid material because it was controversial.

Similarly, the source had passed on information about a visit to Moscow during the presidential campaign by Mr. Page, the Trump campaign aide. But the source told the F.B.I. agents that he — or she — had offered no evidence to support one striking claim in Mr. Steele’s reporting: that Mr. Page had been offered a lucrative brokerage fee in the sale of part of the Russian oil giant Rosneft.

Another of Mr. Steele’s claims that drew wide media coverage was that Mr. Trump’s personal lawyer, Michael D. Cohen, had traveled to Prague during the campaign to meet with Russian agents and discuss the hacking of the Democrats. The inspector general’s report said flatly that the claim was “not true.”

The F.B.I. seriously considered the possibility that some of the claims that reached Mr. Steele were fed into his informant network by Russian intelligence agents, the inspector general said. But some officials were puzzled by the notion that Russia, which was known to have spread falsehoods about Mrs. Clinton, was also planting dirt about Mr. Trump. The bureau appears to have reached no firm conclusion on this critical question. Mr. Steele declined to comment for this article.

Mr. Steele, who had served in Moscow and London for Britain’s MI-6 and was considered a solid Russia expert, had provided information to the F.B.I. for several years before the 2016 election, first on corruption in soccer’s international governing body, known as FIFA, and later on Russian oligarchs, doping by Russian athletes and corruption in Ukraine. Much of the information proved valuable, and the bureau paid Mr. Steele a total of $95,000.

The inspector general found that associates vouched for Mr. Steele’s integrity but questioned his sourcing and the reliability of his reporting. When F.B.I. agents interviewed his former colleagues in British intelligence, they got a mixed review: All agreed that he was truthful, but some found he had shown a “lack of self-awareness” and “poor judgment.”