The MoD said its own police officers were investigating the theft
An investigation is under way after a laptop containing secret data was stolen from the Ministry of Defence.
It was taken from the ministry's headquarters in Whitehall, central London in late November, along with a key used to decode encrypted files.
A spokesman said an investigation by MoD police was ongoing.
Shadow defence cecretary Liam Fox said the theft was "extremely worrying". The incident is the latest in a string of thefts involving MoD laptops.
Figures from the department earlier this year showed that 28 had been lost or stolen between 1 January and 11 May.
And last July, the MoD admitted that 658 of its laptops had been stolen in the past four years.
In just one incident, personal information belonging to 600,000 people considering applying to join the services was lost, including National Insurance numbers and bank details.
The Sun newspaper reported that the latest machine to be taken had been left at the building in Whitehall by a senior RAF officer.
Mr Fox said: "This goes way beyond the careless loss of a laptop or lapses in personal security that we have seen in recent times.
"I'll be seeking clarification from the Secretary of State as soon as possible."