Mr. Comey, whom Mr. Trump fired in May 2017, had previously said the case involving Mr. Page was handled in a “thoughtful, responsible way” during the time he was the director of the agency.

But the inspector general’s report found that, among other problems, the F.B.I. never told the Justice Department that Mr. Page had for years been providing information to the C.I.A. about his prior contacts with Russian officials, a point that might have made his history less suspicious. Over all, the report said the inspector general found “a number of factual representations that were inaccurate, incomplete or unsupported by appropriate documentation” in the wiretap applications.

Mr. Trump seized on Mr. Comey’s admission to attack him.

“So now Comey’s admitting he was wrong,” the president said on Twitter on Sunday. “Wow, but he’s only doing so because he got caught red handed. He was actually caught a long time ago. So what are the consequences for his unlawful conduct. Could it be years in jail? Where are the apologies to me and others, Jim?”

But Mr. Comey noted that the inspector general did not find evidence of the most serious charges of political bias and a “deep state” conspiracy that Mr. Trump and his allies have leveled against the F.B.I. in the years since the 2016 campaign.

He firmly defended the F.B.I., saying the bureau had been proved to be honest and apolitical, but “flawed.”