The government has asked prison staff to make them aware of any "stability issues" that may develop inside jails after delays to prisoners' early release dates, Sky News understands.
It comes after Prime Minister Andy Burnham announced a "pause" to the first tranche of releases, which were due to start in September.
One prison service source suggested Burnham had come into office understandably wanting to prioritise the safety of victims but without thinking of the repercussions for prison capacity if the scheme were to be scrapped.
It's a "mess", they said.
Releases began being prepared at the beginning of the month, according to internal guidance circulated within the probation service and seen by Sky News.
Up to 7000 inmates were set to be freed across 10 tranches, starting on 2 September.
Probation staff have now had their training on the changes paused, and have little further information about the shape of the review and what could change.
The Sentencing Act, which became law earlier this year, introduced an earned progression model which allowed prisoners serving standard sentences in England and Wales to be freed after serving a third of their sentence, rather than half.
The rest would be spent on supervision in the community, using electronic tagging.
More serious criminals, including those convicted of sexual and violent crimes, would be eligible for release from prison after serving half of their sentence, rather than two-thirds, as is typically the case at the moment.
It is believed that this change led to two of the three men convicted of manslaughter over the death of PC Harper being considered for early release from prison.
On Friday, the Police Federation for England and Wales urged the government not to allow prison capacity to "redefine the punishment imposed by the courts", saying any celebration of police officer bravery rings "hollow" when officers see "the very people they risked their lives to arrest being released early... because the prison estate has run out of capacity."
Some sources have suggested that the government could change the eligibility criteria so that more serious criminals are prevented from release, but there are questions over whether this amendment would do enough to curb overcrowding and deliver the numbers.
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The men's prison estate is operating at 97% capacity today, with around 2,000 free spaces. Not quite the 99% capacity it was at two summers ago, with just hundreds of free spaces, but it wouldn't take too long to get back to that point.
In a statement released on Thursday, the prime minister said keeping the public safe would always be his "top priority" and that "no prisoners will be let out under this policy until we have conducted an urgent review".
"Where changes are needed, we will not hesitate to make them," he added.
(c) Sky News 2026: Prison staff asked to warn of any prison unrest due to delays in releases
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