A US federal judge says "investigative missteps" and "government misconduct" may have tainted the Trump administration's criminal case against former FBI director James Comey.

In a 24-page opinion, Judge William Fitzpatrick ordered Justice Department lawyers to release grand jury materials, including transcripts and evidence, to Comey's defence team.

The Justice Department has appealed against the order.

Comey was charged in September after President Donald Trump called for his prosecution and appointed a new federal prosecutor to pursue the case.

Comey, who has long drawn Trump's ire, has pleaded not guilty to the charges, which include obstructing a congressional investigation.

The former FBI chief was fired by Trump during his first term, after Comey led an investigation into Russian election interference in the 2016 US presidential election. Since then, Comey has been a frequent target of the two-term US president.

Comey was accused of lying to lawmakers during a congressional hearing in 2020 about his Russian election interference investigation.

After other federal prosecutors had reportedly declined to pursue the case against Comey, citing a lack of evidence, Trump tapped Lindsey Halligan as the lead prosecutor in the case. She secured an indictment three days later.

An indictment in the US justice system is a formal accusation issued by a grand jury - a group of citizens who examine the merits of evidence - to determine if a case should proceed.

Comey was formally charged in October in a federal court in Virginia, where he pleaded not guilty.

His defence team has not only taken issue with the legality of Halligan's quick appointment to the role of interim US attorney, a process by which she circumvented congressional approval, but also filed motions over how she brought the Comey indictment.

On Monday, Judge Fitzpatrick wrote that he identified at least two statements Halligan made to grand jurors that could be seen as "fundamental misstatements of the law that could compromise the integrity of the grand jury process".

One statement Halligan made, the judge noted, "suggests" that the grand jury "did not have to rely on only the record before them", and that there was "more evidence - perhaps better evidence" that the government had that would be used at trial.

In granting Comey's defence team access to all grand jury materials, Judge Fitzpatrick has also ordered the Justice Department to hand over complete audio recordings of the proceedings.

"The court recognises this is an extraordinary remedy," Judge Fitzpatrick said.

He continued: "Under these unique circumstances, (it) is necessary to fully protect the rights of the accused."