Virginia-based US Magistrate Judge William Fitzpatrick has ordered that grand jury materials be turned over to the former FBI chief's defence lawyers.
Fitzpatrick found that Lindsey Halligan, the Trump-appointed US attorney leading the case, may have made significant legal errors in presenting evidence and instructing grand jurors who were weighing whether to charge Comey - mistakes that could have tainted the case.
"The record points to a disturbing pattern of profound investigative missteps, missteps that led an FBI agent and a prosecutor to potentially undermine the integrity of the grand jury proceeding," Fitzpatrick wrote in his ruling on Monday.
Comey is one of three prominent critics of the Republican president to have been hit with criminal charges by Trump's Justice Department in recent months.
Trump critics have described the charges as a part of a campaign by the president to suppress opposition.
Comey pleaded not guilty after being charged in September with making false statements and obstructing a congressional investigation.
A different judge previously expressed scepticism about the legality of Halligan's appointment, while other courts have raised alarm about political motivations in Justice Department investigations and grand juries have rejected cases tied to Trump's police surge in Washington.
Halligan had not worked as a prosecutor before Trump appointed her in September. She had primarily practised real estate law and represented Trump in civil litigation.
A Justice Department spokesperson declined to comment on the judge's decision.
Prosecutors quickly sought to pause the order to turn over grand jury material and claimed in a court filing that Fitzpatrick "may have misinterpreted some facts".
Comey has sought to get the charges against him dismissed, saying the prosecution resulted from Trump's "personal spite" against him over his criticism of the president and his leadership of an investigation into contacts between Trump's 2016 campaign and Russians.
Trump has threatened to imprison rivals since he first ran for president in 2015.
Comey's indictment was followed by indictments of two other prominent Trump critics - his former national security adviser John Bolton and New York state Attorney General Letitia James.
The charges in these three cases breached a long-standing Justice Department practise of political independence in criminal investigations.
Prosecutors allege that Comey lied to a Senate committee in 2020 when he said he stood behind prior testimony that he had not authorised anyone at the FBI to be an anonymous source in news reports about investigations into Trump and his 2016 rival, Democrat Hillary Clinton.
Fitzpatrick said he would order prosecutors to turn over grand jury materials to Comey's defence for use in a potential future legal motion to have the charges dismissed.
Grand jury materials are typically protected by strict secrecy rules. Fitzpatrick acknowledged that disclosure to the defence was an "extraordinary remedy" but found that Comey had shown a specific need for the records.
"The Court finds the record in this case requires the full disclosure of grand jury materials," the judge wrote.
22°C