A government plan for early release of prisoners to help prisons treat the coronavirus was suspended after six offenders were released in error.
Detainees were mistakenly released from two open prisons in Gloucestershire and Derbyshire.
Officials said the men “returned freely to the prison when asked to do so.”
The prison service said it had now strengthened its processes.
Justice Secretary Robert Buckland previously said that strict controls would take place before detainees are released from the program.
Up to 4,000 prisoners – those who had to be released within two months anyway – were eligible for the early release program across England and Wales.
Officials said on Tuesday that hundreds of people would be released by the end of the week – including 14 pregnant prisoners and mothers of babies among those released.
The prison service attributed the six false releases to “human error” and said the processes were changed to reduce the likelihood of it happening again.
A prison service statement said: “We know of a small number of low risk offenders who have been released from prison under the early release program following an administrative error.
“The men were released too early but were otherwise eligible for the program, and returned to prison in accordance with demand.
“We have strengthened the administrative processes around the program to ensure that it does not happen again. “
Meanwhile, activists have threatened to sue the government unless vulnerable and elderly prisoners are released immediately to protect them from the coronaviruses.
The Prisoners ‘Advice Service (PAS) warned that these detainees were at increased risk of dying or becoming seriously ill if they tested positive for the virus, and called on Mr. Buckland to take urgent action.
Coronavirus cases have been confirmed in almost half of prisons in England and Wales.
A total of 232 detainees tested positive for coronavirus in 60 prisons at 5:00 p.m. BST Wednesday, and 13 detainees died from the coronavirus.