Jack Straw, the justice secretary, has called for an urgent inquiry into the latest government loss of computer data, a disk containing the personal details of 5,000 prison staff.

Although the prison service was informed of the loss in July, Straw, who is the minister responsible, was only made aware of it yesterday when he was contacted by a Sunday newspaper.

The hard disk containing personal details of up to 5,000 staff, including probation workers, was mislaid by a computing firm working for the Ministry of Justice more than a year ago.

According to a letter sent by the firm, EDS, on July 4, the 500GB portable hard disk contained names, dates of birth, National Insurance numbers and prison service employee numbers of about 11% of the UK prison service’s 45,000 workers.

A copy of the letter, entitled Security Incident Interim Report, was obtained by the News of the World, which informed the justice secretary.

“I am extremely concerned about this missing data,” Straw said last night. “I was informed of its loss at lunchtime 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 Prison Officers’ Association condemned what it described as a cover-up and says the loss could cost the taxpayer millions of pounds as staff request to transfer prisons or be relocated to ensure their families’ safety.

Brian Caton, the POA’s general secretary, said: “It’s absolutely abhorrent that those in authority have tried to cover it up. This serious security breach places at risk the security, safety and wellbeing of both themselves and their families.”

The POA national chairman, Colin Moses, added: “This could end up costing the taxpayer millions of pounds. I hope Jack Straw ensures justice is done.”

This is the latest data loss scandal to hit the government, including two computer discs containing the personal details of details of 25m child benefit claimants lost in the post by Revenue and Customs in October last year, and the loss of 84,000 prisoner details by a Home Office contractor last month.

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