A bidder, who bought the camera for £17 on the auction website, discovered photos of terror suspects, their names and fingerprints and even images of rocket launchers and missiles. The 28-year-old from Hemel Hempstead, Hertfordshire, only found the secret images when he downloaded his own holiday snaps from the Nikon Cool Pix device.

He told local police about the find and was shocked when Special Branch officers arrived at his home days later to seize his new purchase.

Officers have made five visits to his home in the last week to quiz him and his family, The Sun newspaper reported.

A foreign office spokeswoman confirmed that the police were investigating but said she could not confirm or deny the intelligence service's involvement in the probe. She refused to comment on reports that the camera was sold by an MI6 agent.

Among the images which are reported to have been found on the camera is a document, marked 'top secret', which gives details of the encrypted computer system used by MI6's agents in the field.

The material found on the camera is reported to be related to Abdul al-Hadi al-Iraqi, a high-ranking al Qaida officer, who was captured by the CIA in 2007. Neil Doyle, author of Terror Base UK, said: These are MI6 documents relating to an operation against al Qaida insurgents in Iraq.