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U.S. and British spy agencies intercepted and stored images from the webcams of millions of likely innocent Yahoo users, including “large quantities” of sexually explicit images, the Guardian reported Thursday -- a revelation the web giant described as “a whole new level of violation.”

A secret program called Optic Nerve appears intended to collate a digital mugbook of sorts, snapping screenshots every 5 minutes or so from user feeds. But the program targeted indiscriminately, regardless of whether the webcam owner was an intelligence target or not.

More than 1.8 million user accounts from around the world were accessed in one six-month period alone.

Image data from the Yahoo accounts was hoovered up between 2008 and 2012 under the program, which was run by U.K. surveillance agency GCHQ with assistance from the National Security Agency (NSA). The report is based on secret documents taken by former NSA contractor Edward Snowden, which have led to international outrage over the stunning extent of oversight of Internet activities.

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The secret documents show that sexually explicit material was an issue the spy agencies wrangled with; they estimate that between 3 percent and 11 percent of Yahoo webcam images contained “undesirable nudity.”

"Unfortunately … it would appear that a surprising number of people use webcam conversations to show intimate parts of their body to the other person,” one document states, according to the Guardian. “Also, the fact that the Yahoo software allows more than one person to view a webcam stream without necessarily sending a reciprocal stream means that it appears sometimes to be used for broadcasting pornography."

Yahoo responded “furiously” when informed of the Optic Nerve program, the Guardian reported, denying knowledge of the program and calling it "a whole new level of violation of our users' privacy that is completely unacceptable."

“We strongly call on the world's governments to reform surveillance law consistent with the principles we outlined in December,” a spokeswoman said.

The GCHQ said it would not comment on intelligence matters, according to company policy. But a spokesman did say that “all of GCHQ’s work is carried out in accordance with a strict legal and policy framework which ensures that our activities are authorized, necessary and proportionate.”

NSA spokeswoman Vanee Vines said the agency did not ask foreign partners to collect data it could not legally collect itself.

"As we've said before, the National Security Agency does not ask its foreign partners to undertake any intelligence activity that the U.S. government would be legally prohibited from undertaking itself," she told the Guardian.