BOSTON — The judge who was set to preside over the trial of reputed gangster James “Whitey” Bulger was removed from the case Thursday by a federal appeals court that found his background as a former federal prosecutor could create the appearance of bias against Bulger.

The 1st U.S. Circuit Court of Appeals ruled that U.S. District Judge Richard Stearns must step down from the case.

Bulger’s lawyers argued that Stearns should be removed because he was a federal prosecutor in Boston in the 1980s. At the time, Bulger was working as an FBI informant while allegedly committing crimes, including murder.

Bulger claims he received immunity for his crimes from another federal prosecutor who worked in the same office as Stearns while Bulger was giving the FBI information on the Mafia.

The appeals court said it believes that Stearns, who twice rejected the defense request to step down, is sincere in his belief that he could be impartial at Bulger’s trial.

But the court said in its ruling, “we are nonetheless bound to conclude that it is clear that a reasonable person might question the judge’s ability to preserve impartiality through the course of this prosecution and the likely rulings made necessary by the immunity claim.”

The decision was written by retired U.S. Supreme Court Justice David Souter, who sits on the federal appeals court in Boston for three months each year.

The Bulger case will now be reassigned to another judge.

Bulger, the 83-year-old former leader of the Winter Hill Gang, is awaiting trial on charges of playing a role in 19 murders.