Harry Roberts who gunned down three unarmed police officers in Shepherds Bush, west London, in 1966. Photo: PA

Police killer Harry Roberts is to be released from prison – despite being warned by a judge that he faced dying in jail.

Roberts, now 78, was jailed for life for the murder of three policemen in an act which shocked the country and was condemned by the judge as “the most heinous crime for a generation or more”.

His upcoming release has sparked fury, with the Police Federation branding it a “betrayal” of the dead officers, while mayor of London Boris Johnson said the public would be “sickened by this news”.

But Deputy Prime Minister Nick Clegg defended the Parole Board, saying it would be a “disaster” to let emotions dictate which prisoners can be released and when.

Roberts was sitting with two accomplices in his van near Wormwood Scrubs prison in west London preparing for an armed robbery when he opened fire on three unarmed officers.

This decision is a slap in the face for the families of the three police officersd

Detective Sergeant Christopher Head, 30, Detective Constable David Wombwell, 25, and Constable Geoffrey Fox, 41, were killed in the attack on August 12 1966.

The triple murder shocked the nation, and 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 shortly, after the Parole Board approved his release.

Steve White, chairman of the Police Federation of England and Wales, said: “I am appalled to learn that police killer Harry Roberts is being released from prison. Let’s not forget, this menace murdered three unarmed police officers in cold blood and it is abhorrent news.

A police cordon at the spot where three unarmed CID police officers were shot dead in Shepherds Bush in 1966. Photo: PA

“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 relive their pain and loss. It will spark fury among everyone in the police family who will feel badly let down.”

He added: “The judge said Roberts should never be released, and that has just been forgotten and thrown in the bin. What kind of message is that sending? It is appalling. This is a betrayal of the police officers who died.”

The Metropolitan Police Federation labelled the decision a “betrayal of policing by the judicial system”.

It said: “It is a scandalous, hurtful and abhorrent decision which opens the door even further for those who have scant regard for law and order.

“Those who place their lives on the line to protect the public deserve better than this terrible outcome.”

Gill Scott-Moore, chief executive of the Police Dependants’ Trust, which gives financial assistance to the families of officers killed in the line of duty, said his release will “bring back painful memories”.

Johnson said: “It is perfectly possible that the Parole Board has seen new evidence, I don’t know. What I do know is that Londoners will be absolutely sickened by this news. 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. To my mind, in the case of the murder of a police officer, life should mean life.”

Asked if David Cameron backed the mayor’s verdict, the Prime Minister’s spokesman said he “very much understands the strength of feeling that this case will provoke” but that it was a decision for the independent body.

Trial Judge Mr Justice Glyn-Jones jailed Roberts for life at the Old Bailey, giving him a minimum 30-year tariff. Sentencing him, he said: “I think it likely that no Home Secretary regarding the enormity of your crime will ever think fit to show mercy by releasing you on licence.

“This is one of those cases in which the sentence of imprisonment for life may well be treated as meaning exactly what it says.”

White said Roberts’s release underlined the urgent need for new legislation to ensure life prison sentences for those who murder police “mean life”.

In May last year, Home Secretary Theresa May said she would introduce legislation to make sure life does mean life for people who murder police officers, and it is expected to come into law next year.