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, and a young offenders institution in south-east London.
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 six men were released mistakenly from Sudbury and Leyhill prisons and Isis young offenders institution in south-east London.
The Prison Service attributed the releases to "human error" and said processes were being changed to reduce the likelihood of it happening again.
A 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 have 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 more than half the prisons in England and Wales.
A total of 255 prisoners tested positive for coronavirus in 62 jails, and 13 inmates have died.
The most recent figures also show coronavirus has affected 138 prison staff across 49 sites, as well as seven prisoner escort and custody services employees.