In June, security documents were left on a train in London.
A handheld MI5 computer thought to contain information about fighting terrorism has been stolen from a house in Greater Manchester.
The palmtop device was stolen on Sunday night by a burglar who got in through an open window of the house.
Police said the files were encrypted so it would be impossible for anyone to access them.
Officers believe it was an opportunist theft and that the house was not targeted. Inquiries are continuing.
Anyone offered the equipment for sale is urged to report it to Greater Manchester Police.
The location of the house, which is believed to have been rented by the security service, has not been disclosed.
A force spokeswoman said: "Shortly after 11.20pm on Sunday 28 September 2008, police were called to a house in Greater Manchester, following reports of a burglary.
"An encrypted handheld computer was stolen. It is believed the offender entered through an open window."
A Home Office spokesman said: "We do not comment on matters of security."
The security services and the government have been hit by a number of security breaches in the past year.
In June, a civil servant lost top-secret documents containing the latest intelligence on al-Qaeda when he left the papers on a seat on the train in London.
The official was later suspended from his job and government officials described it as a "serious" security breach.
The Ministry of Defence has also admitted 658 of its laptop computers have been stolen over the past four years.
Last year, the security service launched a recruitment drive in Greater Manchester as it opened a regional office.
The move was part of a huge expansion of MI5 in the wake of the September 11 terrorist attacks in 2001.
By 2011, it is expected to have 4,000 staff, of whom a quarter will be based in the regions.