The Georgia judge overseeing Donald Trump's trial on charges of trying to overturn his election defeat in the US state said that lead prosecutor Fani Willis can remain on the case, so long as she removes a deputy she had a personal relationship with.

Judge Scott McAfee's ruling was a blow to the Republican former US president, who seeks to unseat Democratic President Joe Biden in a November 5 election. Trump has sought to delay trials in the four criminal cases he faces until after the election.

McAfee's decision caps a tumultuous two months for Fulton County District Attorney Willis, whose romantic relationship with Nathan Wade, the special prosecutor she appointed to lead the case, was disclosed in a January court filing by a Trump co-defendant.

It also ends three months of contentious litigation and evidentiary hearings over the relationship that effectively paused the rest of the case, though McAfee has yet to set a trial date.

Defence lawyers said the relationship posed a conflict of interest and improperly enriched Willis and Wade, who vacationed together while Wade was drawing a government salary.

McAfee found the relationship did not pose a conflict of interest but said it created "a significant appearance of impropriety" that required either Willis or Wade to step aside.

Trump's lawyer Steve Sadow said in a statement that he respected the judge's ruling but believed it did "not afford appropriate significance to the prosecutorial misconduct of Willis and Wade."

Willis' office did not immediately respond to a request for comment.

Trump has pleaded not guilty in all the cases against him. He is accused in the Georgia case of illegally pressuring state officials to overturn his loss to Biden there in the 2020 presidential election.

He has so far been successful in delaying the start of any trial as he seeks to return to the White House. (Reuters)