Andy Burnham has asked his justice secretary to find a way to keep two of PC Andrew Harper's killers behind bars when the early release scheme comes into effect in October.
A government spokesperson said the prime minister has told Alex Norris to "work through further options to keep a greater number of the most serious offenders in prison, including the killers of PC Harper".
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It came just hours after Number 10 reiterated it was "not possible" for the government to exempt just the killers of PC Harper from the early release scheme.
Jessie Cole and Albert Bowers, both convicted of manslaughter of the Thames Valley Police Officer, are set to be eligible to be let out as part of the government's efforts to ease prison overcrowding.
The pair were in the car that dragged PC Harper to his death in 2019 and were given 13-year terms.
Henry Long, the driver of the vehicle, received a 16-year sentence and would not be eligible for release.
It is understood Mr Burnham has been holding calls while on holiday over the past week to discuss how to resolve the issue and is said to be pushing for solutions.
The prospect of Cole and Bowers' early release has drawn widespread criticism, with former officer Chris Bushnell, a colleague of PC Harper who chased down his killers the day he died, accusing the government of "spitting in the faces of police officers".
Mr Bushnell, who witnessed his colleague's body being dragged behind the suspects' vehicle after it was caught in a strap attached to the car, said: "We can't change the verdict. They got their sentences, which were pretty pitiful, to be fair.
"When I first heard a couple of weeks ago there was a chance they could be let out early, I was shaking. It brought everything back."
PC Harper's widow, Lissie, has previously called the potential move "deplorable".
'Absolutely outrageous'
Some 5,000 people are expected to be released early under sentencing changes, down from 6,000 after the exemptions announced as part of an urgent review of the scheme.
In a statement issued by Thames Valley Police Federation, as a petition to keep PC Harper's killers in jail surpassed 880,000 signatures, Mr Bushnell said: "I will never get over it. The guilt. That feeling of 'what if?' Today, I feel pretty much how I felt the day after it happened.
"This news about the sentencing – and the offenders being released early – has brought it all back again."
The former officer, who visits the scene each year to pay tribute and has received counselling and support from the Police Federation, described the incident as "beyond brutal".
"To think they could be released early is absolutely outrageous. I'm angry with them, but I'm also angry with the government. They're spitting in the face of police officers," he said.
An early release also "belittles" the officers who were traumatised by the violent death and "makes them feel worthless", while doing "nothing" to support the service's efforts to tackle crime, he added.
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In response, a Ministry of Justice spokesperson said: "The death of PC Andrew Harper was a horrific crime. Our thoughts are with his family and friends, and we share the anger from the police community that we are in this position.
"Thanks to tireless campaigning by Andrew's family, Harper's Law means that anyone convicted of killing an emergency worker whilst committing a crime now gets a mandatory life sentence. This government has gone further by introducing a presumption that anyone convicted of murdering a police, prison or probation officer is handed a whole-life order."
The statement, which came before news of Mr Burnham's latest intervention, added: "We have been clear that we wanted to go further and block more offenders from getting out sooner into their sentence but could not do so without risking prisons reaching capacity and collapsing.
"If that happened, police could not make arrests, courts could not remand offenders, and there would be unchecked criminality on our streets."
(c) Sky News 2026: Burnham tells justice secretary to find way to stop release of PC Harper's killers
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