A hard drive containing the details of up to 5,000 prison staff has been lost, the government has confirmed.
The names, dates of birth, national insurance numbers and prison service numbers were lost by a private firm EDS as long ago as July 2007.
However, staff only realised the data was missing in July of this year.
Justice Secretary Jack Straw confirmed he has ordered an inquiry into the loss and was trying to establish why he was not told of it by the Prison Service.
The loss of the data by private computing firm EDS was reported to the Prison Service in July, but Mr Straw was not informed of the problem until now, after a letter about the missing disc was passed to the News of the World newspaper.
In a statement, Mr Straw said: "I am extremely concerned about this missing data.
"I was informed of its loss at lunchtime today (Saturday) and have ordered an urgent inquiry into the circumstances and the implications of the data loss and the level of risk involved.
"I have also asked for a report as to why I was not informed as soon as my department became aware of this issue.
"My officials are also in touch with EDS as part of these processes. We take these matters extremely seriously."
The latest disclosure comes just weeks after the details of thousands of criminals, held on a computer memory stick, were lost by a private contractor.
Last October two computer discs containing personal details of 25 million child benefit claimants were lost.
Shadow justice secretary Nick Herbert said: "The records of prisoners have been lost already and now we discover that personal data about prison officers has gone too.
"When was this incompetent government planning to own up to another data disaster - this time one which has put the security of thousands of its own employees at risk - and if, as they claim, they didn't know about this until now, who on earth is actually running the department?"
(BBC)
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