According to the prisons watchdog, offenders at Hollesley Bay prison, Suffolk, have been allowed out on day release without proper restrictions.
Inmates deemed to be high risk at the open prison were not always promptly assessed, inspectors said. In some cases, violent men had been allowed out to stay overnight with their family. One prisoner had been assessed as being a risk to children, yet no restrictions had been applied to him.
"Of concern, and in contrast to much that was happening in the prison, public protection work was not good enough."
The watchdog also found Hollesley Bay's monitoring of mail and telephone contact to be "very poor".
The Prison Service said steps have already been taken to improve police protection. However, the inspection also found Hollesley Bay to be "very safe", with good or reasonably good outcomes for its inmates.