A federal judge suggested on Wednesday that Michael Cohen committed perjury under oath, giving fresh support to the former president Donald Trump’s claims that his one-time personal lawyer and star witness at his upcoming criminal trial in New York is an untrustworthy liar.

Judge Jesse M Furman in Manhattan questioned Cohen’s truthfulness in a written order denying his request for early release from the court supervision that followed his three-year prison sentence for crimes including tax evasion, lying to banks and Congress and violating campaign finance laws.

Furman cited Cohen’s testimony at Trump’s civil fraud trial last October. On the witness stand, Cohen had insisted he was not actually guilty of tax evasion, even though he pleaded guilty in 2018.

Asked if he had lied to the judge who had presided over that case, Cohen said: “Yes.”

“Cohen repeatedly and unambiguously testified at the state court trial that he was not guilty of tax evasion and that he had lied under oath” to the late judge William H Pauley III, Furman wrote.

He said Cohen’s testimony “gives rise to two possibilities: one, Cohen committed perjury when he pleaded guilty before Judge Pauley or, two, Cohen committed perjury in his October 2023 testimony.

“At a minimum, Cohen’s ongoing and escalating efforts to walk away from his prior acceptance of responsibility for his crimes are manifest evidence of the ongoing need for specific deterrence,” Furman wrote as he explained why he left in place the court supervision that is scheduled to end later this year.

Trump has repeatedly attacked Cohen’s credibility, saying he “committed MASSIVE PERJURY, at a level seldom seen on the stand before” when he testified at the civil fraud trial in state court.

Trump lost that case, which involved allegations that he had lied about his wealth on financial statements given to banks and others. A different judge ordered Trump to pay a $454m penalty.

In response to Furman’s findings, Cohen sent reporters a statement from his lawyer, E Danya Perry, in which she said she took exception to the judge’s conclusion, calling it “factually inaccurate and legally incorrect”.

“Judge Furman did not have a front seat to the testimony at the lengthy trial,” she said. Perry added that the trial judge ultimately stated that Cohen “told the truth”.

“And Judge Furman ignores that Mr Cohen has never disputed the underlying facts of his conduct, and also what many of Judge Furman’s own colleagues on the bench have long noted: that defendants often feel compelled to agree to coercive plea deals under severe pressure,” she said. “That is exactly what happened to Mr Cohen.”

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Alina Habba, one of Trump’s lawyers, said in a statement on Wednesday that Furman “confirmed what we already know: that Michael Cohen committed perjury and should be prosecuted. As the Manhattan DA says on their website ‘one standard of justice for all’.”

In his ruling, Furman also decided not to impose sanctions against a lawyer who cited three bogus artificial intelligence-generated legal case citations in a court filing because he was unaware Cohen had plucked them from research he conducted using Google Bard. Perry discovered that the cases were not real and reported it to the judge.

When Cohen pleaded guilty to federal charges in 2018, he said Trump directed him to arrange the payment of hush money to an adult film actor and to a former Playboy model to fend off damage to his 2016 presidential bid. Federal prosecutors decided not to charge Trump at the time. Before Cohen was sentenced, federal prosecutors attacked his credibility.

Last year, Trump pleaded not guilty in New York state court in Manhattan to 34 felony charges alleging that he falsified internal business records at his private company to cover up his involvement in the payouts.

Cohen is a key prosecution witness in Trump’s hush-money criminal trial, which has been delayed until at least mid-April after the defense complained about a last-minute evidence dump from a 2018 federal investigation into Cohen and the hush-money matter.