The Parole Board has confirmed that a panel has directed the release of Harry Roberts, but declined to reveal when the convicted police killer would leave prison.
Police groups have reacted furiously to the decision.
We can confirm that a three-member panel of the board has directed the release of Harry Roberts.
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.
We are unable to comment further on the details of this case.
The chair of the Metropolitan Police Federation said the imminent release of convicted killer Harry Roberts is a "total betrayal of policing by the criminal justice system".
Roberts, who was jailed for life for the murder of three unarmed policemen in 1966, could leave prison within days following the Parole Board's decision.
Reacting to news of Roberts' impending release, John Tully tweeted: "A total betrayal of policing by the criminal justice system.
"This man should never see the light of day again, life should mean life."
The decision to release Harry Roberts is a "slap in the face" for the families of his victims, the head of the police federation said.
In a statement to ITV News, Steve White, Chair of the Police Federation of England and Wales, said he was "appalled" by the Parole Board's ruling, adding that officers felt "badly let down".
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.
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.
It will spark fury among everyone in the police family who will feel badly let down.
This is a betrayal of the police officers who died.
Harry Roberts opened fire on three unarmed police officers after they pulled over his van ahead of an armed robbery near Wormwood Scrubs Prison in 1966.
The murders of PC Geoffrey Roger Fox, 41, Detective Constable David Stanley Bertram Wombwell, 30, and Detective Sargent Christopher Tippet Head, 25 shocked the nation.
Roberts went on the the run for more than 90 days and was eventually found in Hertfordshire following a huge manhunt.
The judge who jailed Roberts described it was "the most heinous crime for a generation or more", and warned that he could spend the rest of his life behind bars.
But Roberts, now 78, could be released within days after approval by the Parole Board.
Police killer Harry Roberts is to be released from prison after more than 45 years behind bars.
Roberts, now 78, was handed a life sentence for the murder of three policemen in Shepherd's Bush, west London, in 1966.
Detective Sergeant Christopher Head, 30, Detective Constable David Wombwell, 25, and Pc Geoffrey Fox, 41, were shot dead without warning while questioning Roberts and two other suspects in a van on August 12, 1966.
His 30-year minimum tariff expired 18 years ago.
He could leave Littlehey Prison in Cambridgeshire imminently after the Parole Board approved his release.
A Ministry of Justice spokesman said: "We do not comment on individuals."
Police staff in England and Wales, including community support officers and fingerprint experts, are to be balloted for industrial action, unions announced today.
The industrial action would be part of a dispute over a 1% pay offer.
The group charged with keeping undercover policing in check should be overhauled immediately, a damning report by inspectors has said.Read the full story ›
The public must be convinced to agree to mass monitoring of emails and phone calls or police risk being unable to trace serious criminals or terrorists, the head of the National Crime Agency (NCA) has suggested.
Keith Bristow, director general of the NCA, told the Guardian that snooping powers in the UK were 'too weak' and that he had to do better to explain why those powers were needed to improve security and that public consent was necessary.
The news follows the Home Secretary's speech at the Conservative Party conference last week in which Theresa May indicated a Tory government could enact the so-called snoopers' charter, giving security and law enforcement agencies access to communications data.
Mr Bristow told the paper:
What we have needs to be modernised ... we are losing capability and coverage of serious criminals. I will and need to be better at explaining why this is important in the world that we live in. If we seek to operate outside of what the public consent to, that, for me, by definition, is not policing by consent ... the consent is expressed through legislation. We operate within the law.
He added police and politicians would have to win "the public consent to losing some freedoms in return for greater safety and security".
Labour would reverse cuts to the police planned for next year if it wins power in May, shadow home secretary Yvette Cooper has said.
The party would also scrap police and crime commissioners (PCC) to pay to protect women's refuges.
In every part of the country refuges and services for women and children suffering from domestic violence are being closed due to the callous attitude of this Government.
Theresa May's failure to act means victims are being left with no support and being abandoned.
Refuges provide vital support for women and children. Local specialist services that have a track record of successfully helping women and children need our support and that's what we will do.
Officers investigating the deaths of two babies at a London hospital are now looking into the death of a third baby in a Cambridge hospital.
The current focus of the investigation is on the production of intravenous (iv) feeds and police say they are not investigating the health trusts involved.
The families have been notified of the investigation.
Police said: "We are aware of a small number of other babies who received this feed, and are working with relevant agencies to speak to those families."
Police were already looking at the deaths of two babies at Guy's and St Thomas' NHS Foundation Trust.
The third baby died at the Rosie Maternity Hospital, part of Cambridge University Hospitals.