Notorious killer Harry Roberts, who murdered three policemen and was warned he faced dying in jail, is to be released from prison within days.
Roberts, now 78, was jailed for life in 1966 and condemned by the judge for "the most heinous crime for a generation or more".
He was sitting in his van with two other men near Wormwood Scrubs Prison in west London preparing for an armed robbery when he opened fire on three plain-clothed officers.
Detective Sergeant Christopher Head, 30, Detective Constable David Wombwell, 25, and Pc Geoffrey Fox, 41, were all killed in the attack on August 12.
Roberts would have hanged for it – but the death penalty had been abolished the year before.
Roberts is expected to leave Littlehey Prison in Cambridgeshire within days after the Parole Board is understood to have approved his release.
But Steve White, chairman of the Police Federation of England and Wales, said he was "appalled" at the decision.
"This menace murdered three unarmed police officers in cold blood and it is abhorrent news," he added.
"This decision by the parole board is a slap in the face for the families of the three police officers he brutally murdered who, once again, are forced to re-live their pain and loss.
"This is a betrayal of the police officers who died."
The killings became known as the 'Shepherd's Bush murders' and the infamy surrounding the triple murderer sparked sick chants among football fans who sang from the terraces "Harry Roberts is our friend – he kills coppers".
Old Bailey judge Mr Justice Glyn-Jones jailed Roberts for life for his crimes, giving him a minimum 30-year tariff, and warning: "This is one of those cases in which the sentence of imprisonment for life may well be treated as meaning exactly what it says."
The current minimum term is 30 years before a killer of police or prison officers can be considered for parole and under the amendment to the Criminal Justice & Courts Bill, a whole-life sentence would not be mandatory.
A Ministry of Justice spokesman said it could not comment on individual cases, but said that the release of life sentence prisoners was directed by the independent Parole Board "once they are satisfied they can be safely managed in the community".
The Parole Board said in a statement: "The decision to release is a matter for the board, which is independent - arrangements and the date of the release are a matter for the Secretary of State for Justice."
Deputy Prime Minister Nick Clegg, when asked on his weekly LBC phone-in about the decsion to release Roberts, said he wanted to defend "valiantly and vigorously" the system which takes such decisions, however unpopular and controversial they were.
"A decision has been taken to release this character after 48 years in prison, it's not about my feelings, it's about how the justice system works. If you want to run the system according to the latest emotion you feel, fine, but that would be a disaster."
But Mayor of London Boris Johnson said Londoners would be "sickened" by the decision.
"They will find it hard to understand how a man who shot dead three police officers in this city in the most horrific fashion can now enjoy the freedom he denied his victims," he said.
"To my mind, in the case of the murder of a police officer, life should mean life."
Additional reporting by PAReuse content