Dozens of people released from jail under the government's emergency prison scheme were freed by mistake.
A Ministry of Justice source said that 37 people were released in error on 10 September, because their offences were wrongly logged under repealed legislation.
This meant these cases were not flagged for exemptions, which were designed to prevent those guilty of certain types of crime from being released.
Five prisoners have not yet been returned to jail but most have been brought back into custody.
A Ministry of Justice spokesperson said: "Public safety is our first priority. That is why we took decisive action to fix the broken prison system we inherited and keep the most dangerous offenders locked up.
"This included blocking the early release of domestic abuse offences such as stalking and controlling behaviour.
"We are working with the police to urgently return a very small number of offenders - who were charged incorrectly and sentenced under repealed legislation - to custody.
"The convictions remain valid with offenders monitored since their release and will soon be back behind bars."
About 1,700 prisoners were released from jails across England and Wales on 10 September in a bid to cut overcrowding - with a similar number also anticipated to be freed in October.
This is in addition to around 1,000 prisoners who are typically freed each week.
Justice secretary Shabana Mahmood announced plans in July to temporarily cut the proportion of sentences inmates must serve behind bars from 50% to 40%.
That came as the Ministry of Justice (MoJ) said overcrowding had pushed jails to the "point of collapse".
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Downing Street said the policy had to be brought in to avoid "unchecked criminality" where the police and courts are unable to lock anyone up.
The policy does not apply to those convicted of sex offences, terrorism, domestic abuse or some violent offences.
But campaigners and charities warned the scheme may come at the expense of women and children's safety, while others raised concerns that it would put a strain on probation services.
Prison figures have warned the move will only buy time for about a year before the same issue could be faced again.
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