Accounting firm Deloitte last night revealed that a laptop containing data on about 100,000 pensioners had been stolen – just hours after the Ministry of Defence warned the details of 100,000 Armed Forces personnel could be lost.

The embarrassing incidents are the latest in a series of alarming data security breaches, which have undermined the Government's attempts to justify the introduction of ID cards.

The MoD confirmed it had launched an urgent investigation into a "potential" data loss after admitting its IT contractor, EDS, could not account for a portable computer hard drive.

Names, addresses, passport numbers, dates of birth, next-of-kin and driving licence details for up to 100,000 Army, Navy and RAF personnel could be on the drive.

In July the MoD admitted 658 laptops had been stolen over four years and 26 portable memory sticks containing classified information had been stolen or misplaced since January.

Tory MP Nigel Evans, who chairs the All Party Parliamentary Group on Identity Fraud, said: "The loss of such important information including bank details, passport numbers and dates of birth will be music to the ears of fraudsters everywhere.

"It is vital that there is a cultural change across the public sector with all professionals aware of their responsibility to protect and manage personal data."

Liberal Democrat Shadow Defence Secretary Nick Harvey said: "This is extremely serious news and the immediate priority must be to ensure the security of all military personnel. However, once that has been established, there must be an urgent investigation into how such a disturbing breach of security could be allowed to happen."

The Deloitte laptop, which contained pensioners' names, National Insurance numbers and salary details, was stolen last month. It was snatched from an employee, but the company said its security measures meant there was only a "very very low" risk of the data being accessed.

Deloitte has notified police and informed its clients, who have contacted all the pensioners whose data was on the laptop.

No addresses or bank details were stored on the machine, and the company claimed in a statement that its employees had been advised to pay close attention to laptops in public places.

"We believe that the likelihood of unauthorised access to the data held on this laptop is remote due to the opportunistic nature of the theft and the security controls," it insisted.