... that can force your phone to record audio

Facebook is working on controversial software which lets it secretly order users’ smartphones to start recording audio whenever they hear inaudible messages hidden in television adverts.

The social network has always denied rumours that it listens into people’s private conversations and analyses what they talk about so it can show advertising that interests them.

But in a patent application published on June 14 this year, Big Zucker’s research division revealed a system which lets it quietly tell people’s mobile phones to capture ‘ambient audio’.

It allows Facebook to conceal ‘a non-human hearable digital sound’ in the audio of a TV ad or other content.

Although people won’t be able to hear this sound, it contains a ‘machine recognizable’ set of Morse code-style sounds which let it tap out a message to your smartphone and order it to begin recording.

The secret sound could be a very high-pitched voice, Facebook wrote, pitched just above the limit of human hearing.