The files included plans in place to protect Queen Elizabeth II
Officials at Heathrow Airport in London are investigating after a USB stick containing confidential data - including the exact route Queen Elizabeth II takes to the airport - was reportedly found in the street.
A total of 76 folders were on the stick, including maps, videos and documents, the UK's Sunday Mirror reported.
None were encrypted or password protected.
The newspaper said it contained details of the security measures in place to protect Queen Elizabeth II and the types of identification needed by those, including undercover police officers, wanting to access restricted areas.
The files revealed routes and other safety measures for UK cabinet ministers and foreign dignitaries, as well as timetables of patrols used to guard against suicide bombers and terror attacks.
Maps of the exact locations of CCTV cameras, tunnels and escape shafts linked to the Heathrow Express are also said to be on the stick, as well as details of ultrasound radar systems used to scan the airport runways and the perimeter fence.
The pocket-sized device was reportedly discovered in the street by an unemployed man who handed it to the Mirror, which then passed it to Heathrow intelligence chiefs.
It is unclear if the security breach had been intentional or due to incompetence, the newspaper said.
A Heathrow spokesperson said: "We have reviewed all of our security plans and are confident that Heathrow remains secure.
"We have also launched an internal investigation to understand how this happened and are taking steps to prevent a similar occurrence in future."
The spokesperson added: "Heathrow's top priority is the safety and security of our passengers and colleagues.
"The UK and Heathrow have some of the most robust aviation security measures in the world and we remain vigilant to evolving threats by updating our procedures on a daily basis."