James Comey, the former director of the FBI who has become a prime nemesis of Donald Trump, admitted on Sunday to being responsible for “real sloppiness” over the handling of surveillance of a Trump campaign adviser.

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He also fiercely defended himself and the bureau against any suggestion of political bias, prompting a new threat, of “years in jail”, from Trump.

Comey, who was fired by Trump as America’s top law enforcement official in May 2017, came under intense questioning on Fox News Sunday, sparring with anchor Chris Wallace over the findings of the inspector general’s report into the FBI’s investigation into Russian interference in the 2016 election.

Comey seized on one of Michael Horowitz’s main conclusions, that there was no evidence of political bias in the investigation, to launch an impassioned critique of how he and his FBI colleagues had been treated by Trump.

“The FBI was accused of treason, of illegal spying, of tapping Mr Trump’s wires illegally, of opening an investigation without justification, of being a criminal conspiracy to unseat a president. All that was nonsense.”

He also had pointed words for Fox News: “Remember, I was going to jail, lots of other people were going to jail. People on this network said it over and over again … The American people, especially your viewers, need to realise they were given false information about the FBI.”

I was overconfident in the procedures that the FBI had built over 20 years. I thought they were robust enough James Comey

That Comey was given the opportunity by Fox News to welcome the absence of any bias finding in the report clearly riled Trump, who intervened with a characteristically breathless tweet. Paradoxically, the president accused Horowitz himself – an independent watchdog with no known political animus – of bias.

Pointing out that the inspector general was appointed by Barack Obama, Trump claimed: “There was tremendous bias and guilt exposed, so obvious, but Horowitz couldn’t get himself to say it. Big credibility loss.”

The president followed up by attacking Comey himself, in loose and intemperate terms. “So now Comey’s admitting he was wrong,” he tweeted. “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?”

Even before Trump’s intemperate intervention, Comey did not get an easy ride. Under persistent questioning by Wallace, he was forced to admit he presided over serious mistakes in the course of applying for permission to place former Trump adviser Carter Page under surveillance.

The IG report gives details of 17 “significant errors and omissions” in the way four applications to the Foreign Intelligence Surveillance (Fisa) court were made from October 2016, the first two on Comey’s watch.

Asked to explain the contrast between the report’s criticism of serious problems in the Fisa process and his earlier defense of the FBI’s actions as “thoughtful and appropriate”, Comey replied that Horowitz “was right, I was wrong”.

“I was overconfident in the procedures that the FBI had built over 20 years. I thought they were robust enough. It’s incredibly hard to get a Fisa and he was right there was real sloppiness. It was not acceptable.”

Comey’s admittance of fallibility, albeit over procedural issues rather than the overriding accusation of bias, is likely to give Trump and his enablers in the Republican party the fuel they need to continue to attack the FBI as the heart of the supposed “deep state”. William Barr, the US attorney general, has already launched his own investigation of the Russia investigation.

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Trump has gone on the warpath on the back of the IG’s report, despite its central finding of lack of evidence of any conspiracy. At a rally this week in Hershey, Pennsylvania, the president falsely claimed the FBI had conducted multiple undercover operations spying on his campaign.

“Look how they’ve hurt people,” he said. “They’ve destroyed the lives of people that were great people, that are still great people. Their lives have been destroyed by scum. OK, by scum.”

Hours before the Fox News Sunday broadcast, Trump hurled further insults, calling Comey a “sleazebag”. The Texas senator Ted Cruz then added reinforcement, telling ABC’s This Week the IG report was “unbelievably damning of the Department of Justice and FBI. The abuse of power that occurred there is stunning.”

Neither the president nor the senator pointed out that two of the four Fisa applications were brought to the court by Trump’s own administration.

Page was an adviser on foreign affairs to the 2016 Trump campaign. Comey admitted Page had been “treated unfairly” in being subjected to wiretapping. The most egregious aspect, he said, had been Page’s name being made public. “He is a United States citizen and it never should have been made public – that was an outrage.”

Facebook Twitter Pinterest Carter Page speaks in Moscow in 2016. Photograph: Dmitry Serebryakov/TASS

Horowitz pinpointed significant problems committed by three teams of FBI investigators. Two errors in particular stand out.

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The report highlights that the Fisa court was not told Page had contact with the CIA even after the FBI had become aware of that link, which would have severely diluted any suspicion of wrongdoing in his dealings with Russia. A justice department lawyer even altered an email to hide this crucial detail.

The report also criticizes the FBI for critically relying on the highly contentious Steele dossier, an investigation conducted as opposition research against Trump that was paid for initially by Trump’s Republican opponents and then by Democrats.

Horowitz found the Steele dossier had played a “central and essential” role in the Fisa application, despite having contained “misstated or exaggerated” intelligence.

Comey told Fox News the Steele dossier had been just “one of a bunch of different facts that were assembled to apply to the court”. But he conceded it had “convinced the lawyers that they had enough to go forward” with a surveillance application.