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Heathrow Airport’s secret security planning has been revealed in files on a memory stick found in a London street. The documents outline routes and safeguards for the Queen, foreign dignitaries and top politicians using Britain’s busiest airport. The USB drive also includes maps showing where CCTV cameras are located, and escape routes for the Heathrow Express railway serving the airport.

Other files describe the ultrasound detection system for protecting the perimeter fence and the runways, and detail the ID requirements for accessing every area of the airport.

The discovery could trigger diplomatic tension between London and Washington about aviation security standards. During the following week, airlines flying to America were instructed to impose tighter security rules because of fears at the Department of Homeland Security about another attack on US-bound jets.

The memory stick was found by a man just 10 miles from Heathrow. Several days later, he took the device to his local library and studied the contents. When he realised the security implications, he handed the USB to the Sunday Mirror.

A security source said: “In the wrong hands this would represent a profound threat in terms of terrorism or espionage. “Aviation security is under the microscope because of the desire by terrorists to bring planes down in a spectacular fashion. Security services would not want this leaked or sold to hostile parties,” according to mirror.co.uk.

The unencrypted files may have been hacked, then downloaded and transferred to the memory stick, which would raise serious concerns about cyber security.  But given the location of the find, close to Heathrow, it is thought more likely that an airport worker had accessed the data and inadvertently lost the USB drive, according to independent.co.uk.

It is believed that whoever lost the memory stick had security clearance to access the data, if not necessarily to take the information away from Heathrow on a portable drive.

The airport has issued a statement about the discovery: “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.

“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.”