BOSTON — A claim by a lawyer for gangster James “Whitey” Bulger that his client was never an FBI informant is a tactic to influence jurors in the upcoming murder trial, federal prosecutors charged in court documents filed Friday.

Bulger, the former leader of Boston’s notorious Winter Hill Gang, is awaiting trial on a racketeering indictment that charges him with participating in 19 murders.

His lawyer has claimed repeatedly that a former federal prosecutor gave Bulger immunity to commit crimes during the 1970s and 1980s, when Bulger was also working as an FBI informant.

After a court hearing on Bulger’s immunity claim last month, attorney J.W. Carney Jr. told reporters that Bulger was never an FBI informant. Carney still insisted that Bulger had an immunity agreement with prosecutor Jeremiah O’Sullivan, who died in 2009.

In court documents filed Friday, prosecutors called Bulger’s claim “mystifying” and said it is contradicted by more than 700 pages of informant reports on Bulger in the FBI’s files.

“Substantively, Bulger’s claim — that he was never an FBI informant, but still had an unlimited, legally-binding license to kill — is both strange and unsubstantiated,” Assistant U.S. Attorney Zachary Hafer, Brian Kelly and Fred Wyshak Jr. wrote in the court filing.

Bulger wants to use the immunity claim as a defense at trial, but prosecutors have asked U.S. District Judge Richard Stearns to rule on the question before trial. Prosecutors have vehemently denied that Bulger ever had any immunity agreement and say O’Sullivan wouldn’t have had the authority to grant Bulger immunity for committing murder.

“Absent that authority, as a matter of law, that immunity claim fails,” prosecutors wrote in the court filing, asking Stearns to rule before trial that Bulger doesn’t have an enforceable immunity agreement.

In an affidavit filed Friday, a Justice Department official who was O’Sullivan’s supervisor when O’Sullivan headed the New England Organized Crime Strike Force, said O’Sullivan never talked about giving Bulger immunity and wouldn’t have had the authority to make any such deal.

Bulger’s lawyers argue that if the judge prevents Bulger from presenting a defense based on his immunity claim, it would deprive Bulger of his right to a fair trial.

“A decision prohibiting evidence of immunity at trial will also limit the defendant’s fundamental right to be tried by a jury of his peers,” Bulger’s lawyers wrote in court documents filed Friday.

“Whether or not the defendant was granted immunity is at the core of this case,” they wrote.

Carney said Friday night: “The federal government has spent decades poisoning the jury pool about James Bulger. It is total hypocrisy for them to complain when I tell the other side of the story.”

Bulger, now 83, was a fugitive for 16 years before being captured in Santa Monica, Calif., in June 2011. He is scheduled to stand trial in June.

The judge hasn’t indicated when he will rule on the immunity issue.