BOSTON — Lawyers for Boston mobster James “Whitey” Bulger are asking a federal appeals court to remove a judge who’s scheduled to preside over Bulger’s trial on murder charges.

The defense is scheduled to argue Tuesday before the 1st U.S. Circuit Court of Appeals in Boston.

Bulger’s attorneys say U.S. District Judge Richard Stearns has a conflict of interest because he was a federal prosecutor in the 1980s, when Bulger was an FBI informant. Bulger now claims he received immunity for his crimes while giving the FBI information on the Mafia, his gang’s main rival.

Prosecutors argue that Bulger has failed to show that any immunity agreement existed. They say Stearns was never privy to matters involving Bulger.

Bulger is charged with participating in 19 murders.