BOSTON — Lawyers for Boston mobster James “Whitey” Bulger say a jury should decide whether his claim that he was granted immunity by federal prosecutors has merit.

Bulger’s lawyers said in documents filed Monday it is not up to a judge to decide whether the 83-year-old Bulger was granted immunity before he goes to trial, but rather it is up to a jury to decide at trial.

The lawyer says allowing a judge to decide the matter is a violation of the separation of powers between the executive and judicial branches.

Bulger, charged in connection with 19 slayings, has claimed he was granted immunity by a former U.S. attorney who has since died.

Prosecutors call the claim “frivolous and absurd” and have asked the trial judge to bar it from trial.