Andy Burnham is facing an unprecedented mass call from police chiefs to block the early release of PC Andrew Harper’s killers, as unions warned his emergency plan to create more space in jails risked causing confusion or leading to prisoners being freed by mistake.

A letter signed by 50 police leaders across the UK, including every chief constable in England and Wales, called on the prime minister to do everything in his power to stop the killers benefiting from a programme designed to free up prison space, saying there was “a profound strength of feeling across policing” that this should not happen.

Burnham said earlier he recognised “the depth of public concern” about the proposed early release of Harper’s killers and announced a change to the early release scheme, intended to exclude prisoners convicted of the most serious offences by freeing up new prison places.

In their letter, released late on Tuesday night, the leaders, who included the Met commissioner, Mark Rowley, Greater Manchester’s chief constable, Stephen Watson, and the National Crime Agency director general, Graeme Biggar, said: “When the state asks people to run towards danger on behalf of the public, they should be able to trust that the justice system will stand behind them when the worst happens.

“Pc Andrew Harper’s case also raises a wider question.

“When someone is sentenced for an exceptionally serious offence, how much confidence can victims, bereaved families and the public place in that sentence if subsequent changes to release arrangements result in an offender being released significantly earlier than originally anticipated?”

The letter, which was signed by police chiefs from forces across England, Scotland, Wales and Northern Ireland, added: “Officers and staff understand that the law must be applied as it stands. But they will question what message this sends about the value placed on their service and sacrifice when they put themselves in harm’s way to protect others.

“Police officers and staff see first-hand the impact serious crime has on victims and their families. They support people through some of the most difficult moments of their lives and ask them to place their trust in the justice system.

“That trust is vital and should not be weakened. We are also collectively concerned that an approach to early release which does not sufficiently distinguish the most serious offending risks undermining public safety, victim confidence and wider confidence in the criminal justice system.”

Earlier, unions had warned that Burnham’s plan to stop the early release of PC Harper’s killers, Jessie Cole and Albert Bowers, and other serious offenders could increase the risk of some more dangerous prisoners being released by mistake.

It was also announced on Tuesday that probation staff had voted overwhelmingly to go on strike due to what they said were unrealistic workloads, in another blow to the proposals announced by the prime minister.

While the vote by members of Napo was not directly connected to the early release plans, the probation officers’ union has previously criticised the proposals as rushed.

After an uproar about the proposed release of Harper’s killers, Burnham said on Tuesday morning that he had instructed the justice secretary, Alex Norris, to find more prison spaces so the pair and other serious offenders would not be freed.

Writing on Facebook, Burnham said places could be provided by removing foreign offenders more quickly, finding a way to release lower-risk prisoners stuck in jail on indeterminate sentences, and “making greater use of the entire prison estate, including using the female estate for male offenders”.

Downing Street had said on Monday that there was no feasible way of exempting just a handful of prisoners from the scheme to block the release of Cole and Bowers, who were sentenced to 13 years for Harper’s manslaughter. Under the scheme, they were due to be freed in January.

Tom Wheatley, the president of the Prison Governors’ Association, said further changes to the scheme could lead to prisoners being wrongly released. “Mistakes are more likely due to having to recalculate sentences against a very tight deadline,” he said.

Wheatley said removing overseas prisoners was likely to be difficult as many would resist this and a number did not have proper documentation. He said victims would also want to be sure that anyone sent back to their home countries would properly serve their sentence there.

Mark Fairhurst, the national chair of the Prison Officers’ Association, said placing men in women’s prisons would require investment and retraining. “Repurposing women’s prisons to accommodate male prisoners is not something that can simply be achieved by changing the sign above the door,” he said.

“There would inevitably be additional staffing, training, security and operational requirements. The physical design and facilities of women’s prisons are not necessarily suitable for a male population, and those differences would need to be properly assessed before any decision was taken.”

The Ministry of Justice said it was disappointed by the Napo strike vote and it would continue to engage with the union. Officials in Norris’s department are understood to be working at full speed on preparing detailed and workable plans to free up prison space by September.

When the early release scheme was announced, there were exemptions for prisoners convicted of rape, serious child sexual abuse and grooming offences but not manslaughter, prompting pressure to block the release of Harper’s killers, including from police and his family.

Officials initially said it would be unrealistic to just block the release of Cole and Bowers and that exempting all manslaughter cases risked not creating enough prison spaces to ease the crisis. But in his Facebook post and speaking to reporters on a later visit to Derbyshire, Burnham said he was aware of the strength of public feeling about the plans.

“It’s my job to speak up for the public on this issue,” he said during the visit. “I take that duty very seriously indeed, and I’m confident that there’s more that can be done, and will be done to address public concern.”

He said on Facebook: “I am increasingly confident that a more focused approach along these lines will allow us to reduce the number of people released early and, in the case of the most serious offenders like those convicted of the killing of PC Harper, prevent their early release altogether.”

Harper, 28, was fatally injured when he got caught in a strap attached to the back of a car and was dragged down a country road as the killers fled the scene of a quad bike theft in Sulhamstead, Berkshire, in 2019.

Cole and Bowers, two of three men sentenced over the police officer’s death, are due to be released halfway through their 13-year sentences, having been convicted of manslaughter in 2020. A petition demanding an intervention has nearly 900,000 signatures.

The early release changes were part of the Sentencing Act passed earlier this year that cut the minimum amount of time many offenders would have to spend in prison.