The Ministry of Defence admitted that 658 of its laptops had been stolen over the past four years - nearly double the figure previously claimed.
The department also said that 26 portable memory sticks containing classified information had been either stolen or misplaced since January.
The Lib Dems condemned the latest security breaches - which happened despite a desperate cross-Whitehall drive to tighten procedures - as evidence of "shocking incompetence".
However, the MoD insisted that its policies were "generally fit for purpose", and said all data losses were fully investigated.
The embarrassing new details were disclosed by ministers in response to questions tabled in Parliament.
Previously the MoD had confessed to 347 laptops being stolen between 2004 and 2007.
But Defence Secretary Des Browne was forced to issue revised figures after "anomalies in the reporting process" were discovered.
The official total is now 658 laptops stolen, with another 89 lost. Just 32 have been recovered.
In a separate response, ministers said that 131 of the department's USB memory sticks had been taken or misplaced since 2004.
Some 26 of those went this year - including three which contained information classified as "secret" and 19 which were "restricted".
Lib Dem frontbencher Sarah Teather said: "It seems that this Government simply cannot be trusted with keeping sensitive information safe.
"It is frightening to think that secret MoD information can be lost or stolen."
"How can they expect us to trust them to keep our personal information safe in their unnecessary and expensive ID card scheme?"
Last month the MoD was heavily criticised by a review of its data procedures which warned that basic security discipline had been forgotten and there was "little awareness" of the danger of losing information.
But a spokeswoman for the department said today: "Any loss of data is investigated fully.
"The recent report on data losses by Sir Edmund Burton found that MoD policies and procedures are generally fit for purpose, but also identified a number of areas where MoD needs to do better in protecting personal data.
"MoD has developed, and is now working through, an action plan to address all of the report's recommendations and bring the department's handling of personal data to an acceptable state."Reuse content