AP file

Boston mob boss James "Whitey' Bulger.

BOSTON — Gangster James "Whitey" Bulger cannot present evidence to a jury about his claim that he was given immunity for future crimes, including murder, a federal judge ruled Monday, calling Bulger's contention that he had a license to kill "beyond the pale."

U.S. District Judge Richard Stearns found that an immunity agreement "cannot as a matter of public policy license future criminal conduct."

"The court concludes that any grant of prospective immunity to commit murder was without authorization and is hence unenforceable under any circumstance," Stearns wrote.

He also wrote, "A license to kill is even further beyond the pale and one unknown even in the earliest formulations of the common law."

Stearns, however, did not immediately rule on Bulger's claim that he received immunity for past crimes.

"Without knowledge of the date of the alleged agreement, however, the court is unable to say whether this determination nullifies defendant's claim of immunity in its entirety (again, assuming proof of its existence)," he wrote.

Stearns gave lawyers additional time to submit written arguments and indicated he will hold a pretrial hearing on Bulger's claim of immunity for past crimes.

Bulger, the former leader of the Winter Hill Gang, is accused of participating in 19 killings. His trial is scheduled to begin June 6.

The ruling was a blow to Bulger's lawyers, who want to use the claim of immunity as their defense during his trial.

They blasted the ruling and said Bulger's right to a fair trial "is at stake by this decision."

Attorneys J.W. Carney Jr. and Hank Brennan said the federal government "has done everything in its power over the past 25 years to cover up the relationship between James Bulger and Federal law enforcement officials."

"The Federal government, including attorneys who worked for the D.O.J. during this period, desperately want to conceal this sordid history from the jury, the victims, and the public," they said in a statement. "Today's decision is another step toward that goal."

Bulger, now 83, was a fugitive for 16 years after fleeing Boston in late 1994 just before he was indicted. He was captured in Santa Monica, Calif., in June 2011.

Bulger, who authorities say worked as an FBI informant while he was committing crimes, claims he received immunity from federal prosecutor Jeremiah O'Sullivan, who died in 2009.

Bulger's lawyers have asked a federal appeals court to remove Stearns from the case.

Carney argues that Stearns has a conflict of interest because he worked as a prosecutor in the U.S. attorney's office in Boston at the same time as O'Sullivan. The 1st U.S. Circuit Court of Appeals has not indicated when it will rule on the request.

In his ruling, Stearns agreed with prosecutors that he should rule on Bulger's immunity claim before trial.

"Resolution of defendant's immunity claim requires an inquiry only into the existence, scope, and validity of the alleged agreement he made with New England Organized Crime Strike Force Chief Jeremiah O'Sullivan," Stearns wrote.

"It does not require the presentation of evidence regarding the commission of any of the nineteen murders or other crimes with which defendant is charged. The government's motion is thus not only appropriately raised, but calls for pretrial resolution."

Stearns noted that Bulger has the right to testify in his own defense at trial, regardless of the court's ruling on his immunity claim. But he also noted that testifying in one's own defense can be limited by rules relating to "materiality and perjury."

Stearns said there has been a "paucity of information" provided by Bulger's defense regarding his claim of immunity. Bulger's lawyers have said only that O'Sullivan — some time before December 1984 — promised Bulger immunity from prosecution for all past and future crimes, up to and including murder.

A spokeswoman for U.S. Attorney Carmen Ortiz declined to comment on the ruling.

Bulger's lawyers have argued that if the judge prevents Bulger from presenting a defense based on his immunity claim, it will deprive Bulger of his right to a fair trial.

"Whether or not the defendant was granted immunity is at the core of this case," the defense wrote in a court filing Friday.

Carney recently stunned prosecutors when he said Bulger was never an FBI informant. In documents filed in court Friday, prosecutors said the claim that Bulger was not an FBI informant, but still had immunity is "both strange and unsubstantiated."

In their statement Monday, Bulger's lawyers said Bulger is prepared to testify "how and why he received immunity" from O'Sullivan "for past and ongoing crimes."