The British government has confirmed that a portable hard drive holding details of up to 5,000 employees of the justice system including prison staff, was lost last year.
Details of employees of the National Offender Management Service in England and Wales were lost by a private firm, EDS, in July 2007.
However, officials realised the data was missing only in July of this year.
Justice Secretary Jack Straw has set up an inquiry into the loss, and is trying to establish why he was not told of it.
The loss of the data was reported to the Prison Service in July, but Mr Straw was not informed of the problem until midday Saturday after a letter about the missing drive was passed to the News of the World newspaper.
The latest disclosure comes several weeks after the details of thousands of criminals, held on a computer memory stick, were lost by a private contractor.
Previous losses:
Nov 2007: 25 million people's child benefit details, held on two discs
Dec 2007: Details of 7,685 Northern Ireland drivers
Dec 2007: Details of 3 million learner drivers lost in US
Jan 2008: 600,000 people's details lost on Navy officer's stolen laptop
June 2008: Six laptops holding 20,000 patients' details stolen from hospital
July 2008: Ministry of Defence reveals 658 laptops stolen in four years.
