A federal appeals court Thursday removed the judge who was to preside over the trial of reputed mobster James "Whitey" Bulger, seeking to avoid any appearance of bias because the jurist is a former federal prosecutor.

In a ruling written by retired Supreme Court Justice David H. Souter, who sits on the appeals court in Boston for three months each year, the court decided that U.S. District Judge Richard Stearns must withdraw from the case and that a new judge be assigned. Stearns was a federal prosecutor in Boston in the 1980s _ the same period Bulger was working as an FBI informant while allegedly committing crimes, including murder.

Bulger's lawyers have argued that he received immunity for his crimes, including killings, from another federal prosecutor who worked in the same office as Stearns while Bulger was giving the FBI information.

Now 83, Bulger is awaiting trial on federal charges in connection with 19 slayings.

In the ruling, the appeals court was careful to separate Stearns' impartiality from the appearance of bias. Souter's decision noted that Stearns' impartiality was not in doubt, but that a reasonable person might question it given the circumstances of his past prosecutorial role.

"Our analysis of ... relevant facts does not question either Judge Stearns' ability to remain actually impartial or his sincerity in concluding that he is not biased against the defendant, nor do we draw any conclusion that he is biased," Souter wrote.

"Despite our respect for Judge Stearns and our belief in his sincerity, 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," the court held, ordering a new judge.

Bulger allegedly cut a wide swath through Boston's underworld until December 1994, when he went underground to avoid arrest on federal charges. He was in hiding for 16 years, 12 of which he was on the FBI's 10-most-wanted list.