The Ministry of Defence confirmed yesterday that an investigation was under way into how instructions for Nato peace-keepers in the Balkans apparently began flashing on the computer screens of a publishing company in London.
Employees were said to have been confronted with military documents setting out the circumstances in which Nato troops could use deadly force, how to control rioting and procedure for handing war criminals to an international tribunal.
Military intelligence officers reportedly visited the company's offices, saying "this is something we are very worried about", and questioned the staff for an hour.
The security blunder was thought to be the result of a "mistake" rather than the work of hackers. Whether the information became more widely available via the internet was unclear.
The restricted, nine-page document was reportedly included in instructions to K-For, the Nato peace-keeping force in Kosovo, entitled Rules Of Engagement for Land Operations.
It set out circumstances in which deadly force could be used, instructed Nato troops to hand war criminals within 48 hours to the International Criminal Tribunal for the former Yugoslavia and covered riot control, media transmissions and who authorises categories of military intervention.
Military chiefs in London distanced themselves from the embarrassment, suggesting the fault lay with another Nato ally. An MoD spokesman said: "We have been made aware of a computer virus being found which contains text apparently relating to military matters and this has been determined to originate from outside the UK. Appropriate investigation measures have been taken."