Andy Burnham is focused on preventing the release of child sexual abuse perpetrators as part of the government’s early release scheme, as urgent talks on which serious criminals could be exempted go down to the wire.

Government ministers are understood to have 24 hours to assess which criminals can be released early to free up prison space, with every exemption weighed up against the threat of the criminal justice system’s collapse.

The prime minister halted the controversial plans to release thousands of killers, rapists and sex offenders from prison in September, intended to ease critical prison overcrowding, during his second week in the role. He is keen to make as many exceptions as the small headroom in the prison estate will allow, the Guardian understands.

Burnham wants to keep child rapists and other perpetrators of serious sexual abuse from being released early after ministers heard the concerns of victims’ representatives and support services who warned that a failure to put in place safeguards for abuse victims will put them at particular risk.

A review of the scheme is expected to conclude within the next 24 hours to give the prison service time to prepare, with a final decision on exemptions still to be made.

But ministers are keenly aware that if no action is taken prisons will run out of space, while exempting some criminals is likely to lead to further calls for others to also be kept behind bars, sources told the Guardian. The former justice secretary David Lammy, who brought forward the act which paved the way for the releases, previously warned that failing to implement it could leave no capacity across jails in England and Wales in November.

The early release scheme – accompanied by a significant expansion in electronic tagging of offenders – is still expected to take place this autumn with an initial group of about 700, despite warnings by probation officers of potential chaos. It is likely to cause a headache for the new prime minister as he faces the Conservative leader, Kemi Badenoch, in the House of Commons for the first time. “If we don’t take any action we will run out of room – it’s not sustainable at all,” said a government source.

It comes after the family of PC Andrew Harper expressed outrage that his killers may be eligible for early release, with his mother, Debbie Adlam, saying the proposed inclusion of two of her son’s killers – Jessie Cole and Albert Bowers – was “another letdown” for their family.

New sentencing laws were passed under Keir Starmer’s government to ease a looming prison overcrowding crisis that ministers said could seize up the criminal justice system. Victims’ advocates are concerned that high prisoner numbers will limit the number of serious criminals who could be exempted.

The former safeguarding minister Jess Phillips, a vocal critic of the scheme, said it was right that exemptions were being considered – but there needed to be “a whole system change with how victims and risk is managed as the sentencing review goes forward” which had to be carried out with victims’ agencies, probation officers and police.

Under the Sentencing Act 2026 which passed in January, from September some serious offenders will be eligible for release halfway through their sentences rather than two-thirds of the way through. Those convicted of less serious crimes can be freed after serving a third of their sentence instead of 40%.

The new law also restricts the use of short-term prison stays, which will be replaced with probation and community orders – a move that some critics have said will put pressure on an already overburdened probation service.

Phillips said any review needed to examine not only the early release scheme but the presumption against short sentences. “Exemptions are an immediate safety crisis, but real in-depth work to make the situation better forever and be one based on risk and rehabilitation must be done,” she said.

Claire Waxman, the victim’s commissioner, met Alex Norris, the new justice secretary, on Wednesday and told the Telegraph she welcomed his acknowledgment that victims had been let down and was “cautiously more hopeful” about progress on answering her concerns.

But prison reform groups, government officials and politicians reacted angrily to Burnham’s decision to pause the scheme, warning that abandoning measures to ease jail overcrowding could lead to the criminal justice system “collapsing under its own weight”.

Ciara Bergman, the chief executive of Rape Crisis England and Wales, said that centres had been supporting “scores” of victims through the “distress and terror of finding out that their rapists and abusers were due to be released … or in some cases already had been”.

She said: “Victim-survivors should never have been put in this situation, not least because so little is known about the factors which make some more like to offend than others, and how sexual violence offenders can effectively be managed and monitored in the community.”