Hundreds of apps and games are monitoring smartphone users through their microphones.
They feature a technology that can even recognise audio when the user puts their phone inside their pocket, or if the apps are running in the background.
Advertisers can then use this information to learn more about consumers and target ads more effectively.
These apps, some of which are targeted at children, use software from a startup called Alphonso, which quietly collects data about people’s TV viewing habits and sells it on to advertisers.
Around 1,000 games and social apps reportedly use the software, with more than 250 of them available to download from Google Play and a smaller number also available from Apple’s App Store.
While Alphonso hasn’t revealed the names of these apps, Pool 3D, Beer Pong: Trickshot, Real Bowling Strike 10 Pin and Honey Quest all feature the technology.