Image caption Harry Roberts was given three life sentences for the killings

A notorious police killer is to be released from prison in a decision described as "scandalous, hurtful and abhorrent".

Harry Roberts, 78, was jailed for life for murdering three unarmed officers in Shepherd's Bush, west London, in 1966.

They were shot when they approached a van containing Roberts and two others, after an armed robbery.

Home Secretary Theresa May said she strongly believed police murderers should be behind bars for life.

London Mayor Boris Johnson called the decision "sickening".

The Metropolitan Police Federation said the decision was a "betrayal of policing" by the judicial system.

It said Roberts' planned release, approved by a Parole Board panel, was a "scandalous, hurtful and abhorrent decision which opens the door even further for those who have scant regard for law and order".

Media playback is unsupported on your device Media caption Tom Symonds reports on a crime that shocked the UK

"Those who place their lives on the line to protect the public deserve better than this terrible outcome," it added.

But Deputy Prime Minister Nick Clegg defended the Parole Board, saying it was not about "feelings" but "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," he said on his phone-in show on LBC.

Roberts could be released from Littlehey Prison in Cambridgeshire within days, although it could take up to three weeks. He will have to adhere to strict conditions.

The murders of PC Geoffrey Fox, 41, Sgt Christopher Head, 30 and 25-year-old Det Con David Wombwell became one of the most infamous crimes of the 1960s, causing public outrage.

Image copyright PA Image caption The men shot dead were (left to right): PC Geoffrey Fox, 41; Det Con David Wombwell, 25; and Sgt Christopher Head, 30

Roberts and two associates - John Duddy and John Witney - had been sitting in a van near Wormwood Scrubs prison when three policemen approached to ask questions.

Roberts shot dead Mr Wombwell and Mr Head, while Mr Fox was killed by another member of the gang.

The officers were killed in front of children playing in the street.

Jungle training

Roberts was on the run for 90 days, using his Army jungle training to camp out in Hertfordshire, where he was eventually arrested.

He has now been in prison for 18 years longer than the 30-year minimum term recommended by the trial judge.

Responding to the Parole Board decision, Home Secretary Theresa May said: "Policemen and women go out to work every day knowing that they might face great danger and they carry out their duties with great courage.

"I strongly believe that anyone who murders a police officer belongs behind bars - and behind bars for life.

"That is why I have made sure the government will change the law so life will mean life for anyone who murders a police officer."

Met Commissioner Sir Bernard Hogan-Howe said his thoughts were with the victims' families.

He added: "Officers of both yesterday and today deserve the full protection of the law when facing ruthless criminals and in this case "life" should have meant what it said."

How does a killer get released from prison?

By Dominic Casciani, BBC News home affairs correspondent

When someone is convicted of murder, a judge gives them a life sentence. But that does not mean they will necessarily die in jail because the sentence has two parts.

Firstly, they are given a tariff: this is the minimum term that the prisoner must serve before they can be considered for release.

The second part of the sentence is the licence: even if they do get out after their minimum term, they are on licence for life - so never legally free. If they step out of line in any way, they could be immediately taken back to prison.

Once a prisoner's minimum term has passed, their suitability for release can be assessed by an independent expert panel from the Parole Board. Its primary function is to protect the public so it cannot release someone unless it is convinced the offender no longer poses a significant risk of serious harm.

The panel assesses evidence of risk including the offender's behaviour in jail, the judge's comments at sentencing and expert medical or psychological reports.

If the offender is released, they may be subjected to strict controls and their behaviour and whereabouts can be monitored by probation services, police and other agencies.

These arrangements will change for police killers under the Criminal Justice and Courts Bill.

The legislation proposes to tell judges that they should normally consider a whole-life term for anyone who murders a police officer - although they will retain their discretion to reduce the final sentence if the specific circumstances of the case justify it.

John Tully, chair of the Metropolitan Police Federation tweeted: "This man should never see the light of day again, life should mean life."

A former inmate called Mark, who spent some time in prison with Roberts, phoned BBC London 94.9 to say there "isn't a bad bone in his body".

He said: "It's terrible for the officers losing their life but the man's done 48 years. I don't think on the day they went out to murder people."

Mark said the murders had happened "in the heat of the moment".

Image copyright PA Image caption The officers were shot after approaching the vehicle Harry Roberts was in

Image copyright Getty Images Image caption The killings provoked outrage among police officers at the time

Mr Johnson said Londoners would 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," said the London mayor.

"To my mind, in the case of the murder of a police officer, life should mean life."

Image copyright PA Image caption The funeral of the officers drew crowds to the streets in 1966

A Ministry of Justice spokesperson said: "The release of life sentence prisoners is directed by the independent Parole Board once they are satisfied they can be safely managed in the community.

"Once released they are subject to strict controls for as long as their risk requires them. If they fail to comply with these conditions they can be immediately returned to prison."

Roberts' accomplices John Witney and John Duddy were both handed the same sentence as Roberts.

John Witney was released on licence in 1991 after serving nearly 25 years, but was found murdered in Bristol eight years later. John Duddy has also died.