A British spy agency collected webcam images from millions of Yahoo users who thought they were engaging in private video conversations, according to a news report based on leaked documents from the former U.S. government contractor Edward Snowden.

Many of the images were sexually explicit, said the report in Thursday’s Guardian newspaper, which said the British agency GCHQ tapped into transmissions between Yahoo users around the world, apparently with some technical help from the U.S. National Security Agency.

The report sparked new outrage among Internet users and companies that provide Internet services. Yahoo reacted with fury, saying in a statement that it wasn’t aware of the British operation, code-named Optic Nerve.

“We were not aware of nor would we condone this reported activity. This report, if true, represents a whole new level of violation of our users’ privacy that is completely unacceptable,” said a Yahoo spokeswoman, who added: “We strongly call on the world’s governments to reform surveillance laws consistent with the principles we outlined in December.”

The American Civil Liberties Union added in a statement: “This is a truly shocking revelation that underscores the importance of the debate over privacy now taking place and the reforms being considered” by U.S. policy-makers.

While it’s unclear how many Yahoo subscribers were affected in the United States or other countries, the Guardian said documents showed the British agency collected webcam images from more than 1.8 million Yahoo users around the world during one six-month period of 2008.

The program lasted at least until 2012, said the newspaper, which noted that GCHQ has no restrictions under British law against collecting images from U.S. citizens without a warrant. The newspaper also said the agency had no technical ability to keep the images of U.S. or British citizens from being collected.

Many of the images collected under the program turned out to be from sexually explicit webcam sessions, according to the newspaper, which quoted one British government document as saying: “Unfortunately … it would appear that a surprising number of people use webcam conversations to show intimate parts of their body to the other person.”

Optic Nerve collected still images from video sessions, in part because of legal restrictions and also to avoid overloading the government’s storage system, the Guardian said.

The newspaper said the program was launched as a way of monitoring known terrorist suspects and to find new targets, and to test the use of automated facial recognition programs.

The British agency built the program with technical help from their U.S. counterparts and some information may have flowed into U.S. surveillance systems, according to the Guardian, but it’s not clear how much information would have been shared.