Yahoo has reacted with outrage after a report that Britain's spy agency intercepted and stored webcam pictures of millions of its users, including many images with sexual content, in what the internet company described as a "whole new level of violation".
US senators also weighed in on Thursday and said British spies and the US agencies with whom the data was shared had shown a "breathtaking lack of respect" for privacy.
Files from Britain's communications spy agency GCHQ leaked by former US National Security Agency contractor Edward Snowden revealed how the Optic Nerve program collected still images of webcam chats regardless of whether individual users were suspects or not, The Guardian newspaper reported on Thursday.
In one six-month period in 2008, the British spy agency collected webcam imagery from more than 1.8 million Yahoo user accounts around the world, The Guardian said.
The data collected, which was available to NSA analysts, contained a significant amount of sexual content and nudity.
"It would appear that a surprising number of people use webcam conversations to show intimate parts of their body to the other person," one intelligence document published by The Guardian said. It went on to say that a study had found that between 3 per cent and 11 per cent of all the images carried "undesirable nudity".
US senators Ron Wyden, Mark Udall and Martin Heinrich said in a joint statement: "We are extremely troubled by today's press report that a very large number of individuals, including law-abiding Americans, may have had private videos of themselves and their families intercepted and stored without any suspicion of wrongdoing.
"If this report is accurate, it would show a breathtaking lack of respect for the privacy and civil liberties of law-abiding citizens."
They promised to investigate the activity as part of an review of surveillance programmes. "It is becoming clearer and clearer that more needs to be done to ensure that 'foreign' intelligence collection does not intrude unnecessarily on the rights of law-abiding people or needlessly undermine the competitiveness of America's leading industries," the senators added.
Yahoo, which was apparently chosen because its webcam system was known to be used by GCHQ targets, expressed horror at the reported surveillance.
"We were not aware of nor would we condone this reported activity," a spokeswoman for the US technology firm said in a statement. "This report, if true, represents a whole new level of violation of our users' privacy that is completely unacceptable.
"We are committed to preserving our users' trust and security and continue our efforts to expand encryption across all of our services."
Leaked GCHQ documents from 2008 to 2010 explicitly refer to the surveillance program, although T he Guardian said later information suggested it was still active in 2012.
The data was used for experiments in automated facial recognition, as well as to monitor existing GCHQ targets and discover new ones, the British paper said.
The program reportedly saved one image every five minutes from a webcam user's feed, partly to comply with human rights legislation and partly to cut down the sheer amount of data being collected.
GCHQ analysts were able to search the metadata, such as location and length of webcam chat, and they could view the images where the username was similar to a surveillance target.
Additional reporting by Associated Press