Inmates at the first UK prison to be transferred from the public to private sector were locked in their cells for almost a full day after a set of keys fitting every cell door went missing.
Staff at Birmingham Prison, which houses 1,450 prisoners and was taken over by private security firm G4S earlier this month, noticed the keys were missing on Tuesday.
It is not known if the keys have been found, or what action has been taken by the prison, but the firm insisted there was no risk to public safety.
A G4S spokeswoman said: "As a rule, we do not comment on security matters.
"However, all prisons have well-established contingency plans to deal with incidents of this nature.
"Contingency measures are being followed at HMP Birmingham and there is no risk to public safety."