The Guardian newspaper reported Thursday that U.K. intelligence agency GCHQ captured and stored still webcam images of millions of Yahoo users. The captured images included substantial quantities of sexually explicit material.

According to the report, the program is called Optic Nerve and started in 2008. During a six month period, the program collected webcam stills from more than 1.8 million Yahoo user accounts.

A Yahoo spokeswoman said in an emailed statement: “This report, if true, represents a whole new level of violation of our users’ privacy that is completely unacceptable.”

A problem for the spies was sexually explicit images. “Unfortunately there are issues with undesirable images in the data. It would appear that a surprising number of people use webcam conversations to show intimate parts of their body to the other person,” reads a passage from the report.

Because Yahoo allows multiple people to view a webcam stream, the service is sometimes used for broadcasting pornography.