A government scheme to release prisoners early to help jails deal with coronavirus has been suspended after six offenders were freed by mistake.
The inmates were mistakenly let out of two open prisons in Gloucestershire and Derbyshire.
Officials said the men “returned compliantly to prison when asked to do so”.
The Prison Service said it had now strengthened its processes.
Justice Secretary Robert Buckland had previously said rigorous checks would take place before inmates were let out on the scheme.
Up to 4,000 prisoners – those who were due to be freed within two months anyway – were eligible for the early release programme across England and Wales.
Officials said on Tuesday that hundreds would be freed by the end of the week – with 14 pregnant prisoners and mothers with babies among those released.
The Prison Service attributed the six mistaken releases to “human error” and said processes were being changed to reduce the likelihood of it happening again.
A Prison Service statement said: “We are aware of a small number of low-risk offenders who were released from prison under the temporary early release scheme following an administrative error.
“The men were released too early but were otherwise eligible under the scheme, and returned compliantly to prison when asked to do so.
“We have strengthened the administrative processes around the scheme to make sure this does not happen again.”
Meanwhile campaigners threatened to take legal action against the government unless vulnerable and elderly prisoners are immediately released to protect them from contracting coronavirus.
The Prisoners’ Advice Service (PAS) warned such inmates were at an increased risk of dying or becoming seriously ill if they test positive for the virus, and called on Mr Buckland to take urgent action.
Coronavirus cases have been confirmed in almost half of the prisons in England and Wales.
A total of 232 prisoners had tested positive for coronavirus in 60 jails as of 17:00 BST on Wednesday, and 13 inmates have died with coronavirus.