WAKEFIeLD’s maximum security prison was put on high alert after a guard went home with a set of keys.
An investigation is now underway at HMP Wakefield after the blunder on Tuesday night.
The Prison Service has confirmed the set of keys were taken out of the jail that holds around 750 notorious criminals.
It is believed the set included conventional and magnetic keys.
A Prison Service spokesman said he could not comment on whether all the locks at the jail would now have to be changed following the breach.
He said: “A member of staff at HMP Wakefield inadvertently took their keys out of the prison for a short period. The incident was investigated as a matter of urgency, which included a forensic investigation of the keys.
“Commenting on a relock of a prison is a potential breach of security.”
The prison houses mainly serious sex offenders, including some with the highest possible security classification.
Among the notorious criminals who have been held at Wakefield are Soham killer Ian Huntley, crossbow cannibal Stephen Griffiths and Milly Dowler’s murderer, Levi Bellfield.
Other prisons have had to pay hundreds of thousands of pounds to replace keys, which are usually left on site.
It is estimated that the cost of replacing locks and keys at the Love Lane prison could run to as much as £1.5m.