Fulton County District Attorney Fani Willis must step aside from the Georgia election interference case against Donald Trump or remove the special prosecutor with whom she had a romantic relationship before the case can proceed, the judge overseeing it ruled on Friday.

Fulton County Superior Court Judge Scott McAfee did not find that Willis's relationship with special prosecutor Nathan Wade amounted to a conflict of interest that should disqualify her from one of four criminal cases against the former Republican president.

However, the judge said, it created an "appearance of impropriety" that infected the prosecution team, and he questioned the truthfulness of Willis and Wade's testimony about the timing of their relationship.

"As the case moves forward, reasonable members of the public could easily be left to wonder whether the financial exchanges have continued resulting in some form of benefit to the District Attorney, or even whether the romantic relationship has resumed," the judge wrote.

"Put differently, an outsider could reasonably think that the District Attorney is not exercising her independent professional judgment totally free of any compromising influences. As long as Wade remains on the case, this unnecessary perception will persist."

A spokesperson for Willis did not immediately respond to a text message seeking comment on Friday.

'Continue to fight'

A lawyer for Trump said they respect the court's decision but believe the judge "did not afford appropriate significance to the prosecutorial misconduct of Willis and Wade."

"We will use all legal options available as we continue to fight to end this case, which should never have been brought in the first place," defence lawyer Steve Sadow said.

Willis hired Wade to lead the team to investigate and ultimately prosecute Trump and 18 others accused of participating in a wide-ranging scheme to illegally try to overturn Trump's narrow loss to Democrat Joe Biden in Georgia in the 2020 presidential election.

Willis and Wade testified at a hearing last month that they had engaged in a romantic relationship, but they rejected the idea that Willis improperly benefited from it, as lawyers for Trump and some of his co-defendants alleged.

The judge wrote that there was insufficient evidence that Willis had a personal stake in the prosecution. But he condemned what he described as a "tremendous" lapse in judgment and the "unprofessional manner of the District Attorney's testimony."

McAfee said he was unable to "conclusively establish by a preponderance of the evidence" whether Willis and Wade began dating before or after he was hired as special prosecutor.

"However, an odour of mendacity remains," the judge wrote. He said "reasonable questions" about whether Willis and Wade testified truthfully about the timing of their relationship "further underpin the finding of an appearance of impropriety and the need to make proportional efforts to cure it."

Even so, he said, dismissal of the case was not the appropriate remedy to "adequately dissipate the financial cloud of impropriety and potential untruthfulness found here."