Fifty police chiefs have signed a letter urging Andy Burnham to use “every lawful option” to prevent the early release of PC Andrew Harper’s killers.

In the open letter to the prime minister, coordinated by the National Police Chiefs’ Council, policing and law enforcement leaders also called for more protections for victims’ families and communities when offenders’ release arrangements change.

PC Harper, 28, was killed after being dragged behind a car as three teenage suspects fled a quad bike theft in Berkshire in 2019.

Two of his killers were due to be freed in January under plans to release about 5,000 prisoners early to ease overcrowding in prisons in England and Wales. The proposals prompted concern from the public and PC Harper’s family, who said they were “more angry than ever”.

On Tuesday, the prime minister said he understood the “depth of concern across the country” over the case.

He said he had asked Justice Secretary Alex Norris to develop plans to increase prison capacity and keep more serious offenders in custody, adding that he was “increasingly confident” the men would not be freed early. In their letter, police chiefs and other law enforcement leaders welcomed the intervention but said the case raised broader questions about the government’s efforts to tackle prison overcrowding. They called on the prime minister to “establish an additional public protection safeguard” for the most serious offenders.

Those who signed the letter included Met Commissioner Sir Mark Rowley, Greater Manchester Chief Constable Stephen Watson and National Crime Agency Director General Graeme Biggar.

Their letter 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 question extends “far beyond policing”, they said, adding that “families across the country are now facing the distress of learning that offenders may be released earlier than they understood when sentence was passed”.

He told ITV News: “Andy Burnham’s position seems to have changed on an almost daily basis, he is re-traumatising and disrespecting victims and even today we don’t know how the government are going to go about doing this.

“We also don’t know what the position is with domestic abusers or with people who have committed indecent assault against children.

“We Conservatives are clear, these people should all stay in prison to serve their sentence and we would support emergency legislation the day that parliament returns to make sure that happens.”

Norris, the new justice secretary, has been asked to fast-track proposals to create “more headroom” in the prison system, including through deporting more foreign offenders, finding a solution for people on indeterminate sentences and using cells previously holding female inmates to accommodate male offenders.