Drugs have gone missing from a prison healthcare centre, it has emerged.
Staff at HMP Norwich realised a drugs trolley was unlocked and the medication had disappeared in the early hours of 3 December, according to a prison report.
A Ministry of Justice (MoJ) spokesman said an investigation into what happened was currently taking place.
The MoJ added it could not say what drug had been removed from the trolley. It said: "We will take action against anyone found to have been involved."
An officer observed a prisoner walking around the landing near the centre at 00:10 GMT, according to the prison report.
"He escorted him back to his dormitory where he found the gate was unlocked," the report said. The officer then conducted a search of the cell before locking him up, it adds.
Class A drugs intercepted
At the same time, nursing staff discovered the drugs trolley kept in a side room in the centre was unlocked.
"On a further check it was established that a quantity of medication was missing and unaccounted for," the report stated.
NHS England appointed Virgincare to run the health centre in April 2014.
Earlier this year, inspectors found that Class A drugs were intercepted at the prison. Their report revealed that a member of staff was sentenced for "illegal provision of drugs to prisoners for cash".
The Independent Monitoring Board found that healthcare had "improved after an uncertain start to the new contract with Virgincare", but said there were "too many agency staff" running the centre.
A spokesman for Virgincare said: "Following an incident at HMP Norwich, we are currently taking part in a joint investigation into the circumstances."