Heathrow Airport has launched an internal investigation after a memory stick containing top-secret security information concerning the Queen was found in the street.
The files contained maps, videos and details about measures to protect Her Majesty when she uses the airport.
The USB stick, found by an unemployed man in Queen's Park, North-West London, also included routes and safeguards for Cabinet Ministers and foreign dignitaries, according to Daily Mail.
Files revealed every type of ID required to access restricted areas - and a timetable of patrols used to guard against suicide bombers and terrorist attacks.
Last night, security experts warned that the files may have been copied for circulation on the "dark web".
The stick also contained details of the radar system used to scan runways and the perimeter fence.
The shocking security lapse comes just weeks after Britain's terror threat was raised to critical following the Parsons Green Tube terror attack.
The maps lay bare details of the airport's Royal Suite, used by the Queen, Cabinet members and overseas dignitaries.
It is hidden from view in Terminal 5 and guests are driven directly to it.
But the memory stick holds images of the route leading up to the suite and satellite images with the location of nearby checkpoints.
A Heathrow spokesman said: "We have reviewed all of our security plans and are confident that Heathrow remains secure.
"We have also launched an internal investigation."