Yahoo webcam images from millions of users intercepted by GCHQ

• Optic Nerve program collected Yahoo webcam images in bulk
• 1.8m users targeted by UK agency in six-month period alone
• Yahoo: 'A whole new level of violation of our users' privacy'
• Material included large quantity of sexually explicit images

The GCHQ program saved one image every five minutes from the users' feeds. Photograph: Chris Jackson/Getty Images

Britain's surveillance agency GCHQ, with aid from the US National Security Agency, intercepted and stored the webcam images of millions of internet users not suspected of wrongdoing, secret documents reveal.

GCHQ files dating between 2008 and 2010 explicitly state that a surveillance program codenamed Optic Nerve collected still images of Yahoo webcam chats in bulk and saved them to agency databases, regardless of whether individual users were an intelligence target or not.

In one six-month period in 2008 alone, the agency collected webcam imagery – including substantial quantities of sexually explicit communications – from more than 1.8 million Yahoo user accounts globally.

Yahoo reacted furiously to the webcam interception when approached by the Guardian. The company denied any prior knowledge of the program, accusing the agencies of "a whole new level of violation of our users'privacy".

GCHQ does not have the technical means to make sure no images of UK or US citizens are collected and stored by the system, and there are no restrictions under UK law to prevent Americans' images being accessed by British analysts without an individual warrant.

The documents also chronicle GCHQ's sustained struggle to keep the large store of sexually explicit imagery collected by Optic Nerve away from the eyes of its staff, though there is little discussion about the privacy implications of storing this material in the first place