More than 100 USB memory sticks, some containing secret information, have been lost or stolen from the Ministry of Defence since 2004, it has emerged.

The department also admitted that more than 650 laptops had been stolen over the past four years - nearly double the figure previously claimed.

The Liberal Democrats condemned the latest security breaches as evidence of "shocking incompetence".

But the MoD insisted its policies were "generally fit for purpose".

Previously the MoD had confirmed that 347 laptops were stolen between 2004 and 2007.

The Mod said it has no idea on when, where and how the memory sticks were lost.

Defence Secretary Des Browne issued 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 121 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".

BBC security correspondent Frank Gardner said the incident was "embarrassing" for the MoD as they had no idea how or when they had been lost or stolen.

Liberal Democrat MP Sarah Teather received the information after tabling a question in parliament.

Ms Teather said: "It seems that this government simply cannot be trusted with keeping sensitive information safe.

"This shows a shocking degree of incompetence."

Shadow defence secretary Liam Fox said: "To treat national security in such a cavalier fashion is unforgivable."

A Ministry of Defence spokesman said any loss of data was subject to a full inquiry and measures were being put into place to improve data protection.

This is the latest in a series of data loss incidents:

- November 2007 - Revenue and Customs officials lost the personal details of 25 million people

- June 2008 - A computer was stolen from the office of Communities Secretary Hazel Blears and files on counter-terrorism were left on a train

- January 2008 - The MoD revealed that one of its laptops - containing the details of 600,000 people - was stolen from a car

Ms Teather added: "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.

'Action plan'

But an MoD spokeswoman said officials were taking the situation very seriously: "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."

Since the Burton report in June 2008 the MoD has recalled 20,000 non-encrypted laptops and are now encrypting them.

So far half have been through the process. About 2,000 are unable to be encrypted so have been taken out of service.