Georgia’s Court of Appeals on Thursday disqualified the Atlanta prosecutor who brought an election interference case against Donald J. Trump and his allies, a surprise move that threw the entire case into disarray.

A three-judge panel reversed the decision of the trial judge, who in March had allowed Fani T. Willis, the district attorney of Fulton County, Ga., to keep the case, despite revelations about a romantic relationship she had with the lawyer she hired to run the case.

While the decision is likely to be appealed, if it stands, it could doom the entire case. There are currently charges against Mr. Trump and 14 of his allies.

This is a developing story. Check back for updates.