Serial killer Donald Neilson, who was known as the Black Panther, has died in hospital, prison sources said.
Neilson was given four life sentences in 1975 and was one of a small group of notorious prisoners who were told they would spend the rest of their lives behind bars.
The 75-year-old was taken to hospital from Norwich prison with breathing difficulties on Saturday and was pronounced dead at about 6.45pm yesterday, the Prison Service said.
A Prison Service spokesman said: "HMP Norwich prisoner Donald Neilson was taken to outside hospital in the early hours of Saturday December 17 with breathing difficulties.
"He was pronounced dead there at approximately 6.45pm on Sunday December 18.
"As with all deaths in custody, the independent Prisons and Probation Ombudsman will conduct an investigation."
In June 2008, a High Court judge ruled the notorious killer must never be released from prison.
Neilson, who murdered heiress Lesley Whittle in 1975 and also shot dead three sub-postmasters during armed robberies, had applied for the setting of a minimum jail term which would have given him a chance of parole.
But Mr Justice Teare, sitting in London, announced that Neilson's "whole life" tariff must remain.
He said: "This is a case where the gravity of the applicant's offences justifies a whole life order."
The judge rejected argument on behalf of Neilson, who was sentenced to four terms of life imprisonment at Oxford Crown Court in 1976, that the sentencing "starting point" should be one of 30 years.
Neilson, a jobbing builder, kidnapped 17-year-old Lesley Whittle from her home in Shropshire, leaving a ransom demand for £50,000.
Her body was later found in an underground drainage system hanging from the bottom of a ladder to which Neilson had secured her by the neck with wire.
Mr Justice Teare said that between February and November 1974, Neilson - who lived in Bradford - "shot and killed three sub-postmasters in the course of armed robberies of their premises".
He added: "The trial judge said that the applicant (Neilson) never set out without a loaded shotgun or other loaded weapon and that he never hesitated to shoot to kill whenever he thought he was in danger of arrest or of detection."
The fourth murder, committed between January 3 and March 7 1975, was that of Lesley Whittle.
Mr Justice Teare said: "The three murders of sub-postmasters involved a substantial degree of premeditation because the applicant took with him a loaded firearm which he was prepared to use.
"They were committed for gain. The victims were particularly vulnerable because of their occupation.
"The manner in which the young girl was killed demonstrates that it too involved a substantial degree of premeditation or planning. It also involved the abduction of the young girl."
The judge said that there "are and were no mitigating features".
Mr Justice Teare added: "The trial judge said that the applicant's sentence of life imprisonment must mean life and that if he ever were released from prison it should only be on account of great age or infirmity."
It was plain from the sentencing remarks of the trial judge that Neilson was "ruthlessly prepared to shoot to kill if he considered such action necessary".
The location and manner of Lesley Whittle's death "indicates that she must have been subjected by the applicant to a dreadful and horrific ordeal", he added.