(PA) -- The Sunday Mirror says the USB stick – which was not encrypted and had 2.5GB of data – was discovered by a member of the public in Ilbert Street in Queen’s Park, west London, and handed to the paper.

It reportedly contained files revealing information such as security measures used to protect the Queen at the airport, the types of ID needed to access restricted areas and the locations of CCTV cameras and tunnels linked to the Heathrow Express.

A spokeswoman for the airport said security plans had been reviewed in light of the incident.

She said: “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.

“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.”

(Photo: PA)