More than 830 military laptops - nine of which contained information classified as top secret or secret - have been stolen or lost in the past five years, the Ministry of Defence admitted for the first time yesterday.

Another 153 desktop computers have also gone missing over the same period, although only one of those held secret material.

The disclosure follows a detailed security review ordered earlier this year, when it was believed that 503 MoD laptops and 23 desktops had been stolen since 1998.

The latest admission means that up to 1000 computers have gone missing and the national and personal security implications are far greater than first thought.

Hard drives of the missing computers contained details of Britain's deployment plans in advance of the 1991 Gulf War, contact information for the UK's top military commanders and highly classified data on the new US-designed Joint Strike Fighter (JSF).

Another drive contained a comprehensive threat assessment for possible terrorist attacks on Heathrow, including the 62 most likely launch-points for anti-aircraft missiles targeting passenger jets on approach or take-off.

The loss of the laptop containing JSF project information in 2001 prompted the US to make an official complaint about the effectiveness of British security procedures.

Despite an immediate security review, and the issue of new advice to officers to minimise the risks while travelling, an estimated further 900 laptops have gone missing since then.

Encryption of all sensitive data was ordered from January 2007, yet a laptop stolen in Birmingham late last year - with personal information of up to 600,000 serving RAF, Royal Navy and Royal Marines personnel and recruit applicants - was not protected. Critics at the time described its contents as "a potential terrorist goldmine".

According to Peter Wood, from penetration testing company First Base Technologies, once a laptop has fallen into the wrong hands, accessing sensitive data is relatively easy. He said: "Ninety per cent of stolen laptops are probably accessible within 10 minutes, and even many of those with more sophisticated levels of encryption can still be accessed within three hours."

The MoD said: "The loss of any data is treated with the utmost seriousness. Personnel are no longer allowed to remove laptops or hard drives from secure locations and all information is encrypted."