The killing of PC Andrew Harper in 2019 still has the power to shock. Here was a young officer carrying out his duty by investigating a burglary in Berkshire. There were three teenagers involved, and when PC Harper and his colleague tried to apprehend them, the former ended up being dragged behind a car for a mile.

The killers were acquitted of murder but convicted of manslaughter. It was a crime so heinous, and their remorse so lacking, that it led to the introduction of “Harper’s Law”, which imposes a life sentence for anyone convicted of killing an emergency worker.

Unfortunately, but in keeping with constitutional convention, that law cannot act retrospectively; and now there is the repulsive prospect of two of the killers becoming eligible as soon as next June for an earlier release from their jail time under legislation recently passed to reduce prison overcrowding.

The family of PC Harper have been rightly horrified by this development. His widow speaks for many: “It is about whether justice is something we truly believe in, or something that can be negotiated away when it becomes inconvenient."

It surely feels like a denial of justice that a punishment duly handed down can be almost arbitrarily reduced, not because of some fresh evidence or other consideration, but simply because the state has so mismanaged things that there are not enough cells available.

More alert to public opinion than Sir Keir Starmer, Andy Burnham has immediately paused the policy and is reviewing it. He wants to ensure that “everything has been looked at before this policy proceeds”.

The pause he has ordered is lawful in the sense that ministers can determine the timetable for its implementation – but any substantive change in its terms would require fresh drafting, and a return to parliament in September, at the earliest.

More pressing still is the fact that HM Prison and Probation Service has to undertake risk assessments and other checks before releases, which take time; and the whole system is in danger of collapsing through overcrowding again before Christmas.

It is certainly an appalling indictment of the state of the criminal justice system – and there are other disturbing examples emerging of what appear to be unfairness to victims of serious sexual abuse.

Invariably, when ministers are taken to task over this, as they were in the emergency release programme two years ago, they blame their Conservative predecessors. In this, they are now only partially correct.

It is undeniable that, after 14 years in power, the Conservatives only added about 500 spaces to the prison estate. But over the same span, they also systematically boosted the prison population to breaking point. Under successive Tory lord chancellors, the law and sentencing guidelines were inexorably toughened.

Everything that could be done was done, but with minimal investment in the prisons: longer minimum custodial sentences for all levels of crime; prison prioritised over community and other punishments; later eligibility for early release; more whole life orders and fewer tariff reviews for lifers.

These went down well at party conference and met a public thirst for retribution, and each individual measure might have had merit (such as tougher sentences for house-breaking, for too long considered a minor inconvenience); but the cumulative effect, exacerbated by “austerity” cuts to budgets, was to overwhelm the prison and probation services.

All of that is an inescapable, unforgivable fact. But something does seem to have also gone wrong in Labour’s own legislation, the Sentencing Act, which received royal assent in January.

It implemented the eminently sensible and considered recommendations made in the review of the system by David Gauke, a former Conservative justice secretary, but with some very unfortunate consequences for certain high-profile cases. This suggests that the lines drawn around certain types of crime may now be too lenient.

As Mr Burnham has no doubt also now discovered, the act also makes no allowance for a lord chancellor or justice secretary to review individual cases, although the timings for implementation of the act can be changed. To keep the two killers of PC Harper incarcerated after the existing act eventually comes into force and other inmates are being set free, would break the law.

So unless the law is changed rapidly, a number of those who really should remain behind bars, as the courts and parliament intended, will in due course be at liberty, even if tagged and on probation. Bu there is now an opportunity for Mr Burnham to respond to public disquiet and make some changes to the act as well as delay its application – though the pressure on space will continue to intensify.

At this point, Mr Burnham could usefully take the opportunity to end the inhuman treatment of people in prison limbo under the old IPP (imprisonment for public protection) regime, and also speed the deportation of foreign offenders. That would certainly create the time and space for a more orderly reassessment of the act’s shortcomings, and avoid some shameful releases.

The prime minister has at least signalled that he has seen the problem, and wants to solve it, even if there’s not much he can practically do about it right now. Here, as elsewhere, he is bumping into the limits of prime ministerial power.