The USB device contained unencrypted files on the security measures used by the airport to protect the Queen, cabinet ministers and foreign dignitaries.

Information also included timetables of the police patrols that guard against potential terrorist attacks, as well as maps of all CCTV camera locations and tunnels and escape shafts linked to the Heathrow Express.

The memory stick was reportedly found in west London by an unemployed man who later turned it over to the Sunday Mirror newspaper, which then alerted Heathrow to the security breach.

Heathrow said in a statement: “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. Heathrow’s top priority is the safety and security of our passengers and colleagues.”