Facebook has admitted using contractors to listen to and transcribe recordings of users without their knowledge.

The tech giant told Sky News it had now followed Apple and Google in stopping the practice, which was first reported after contractors who worked on the project spoke to Bloomberg.

"Much like Apple and Google, we paused human review of audio more than a week ago," a spokesman said.

While the only recordings used were taken from people who had opted in to having their voice transcribed to help the company improve its tools, they had expected to have only been heard by an AI.

Instead, Facebook was until recently using contractors to review audio, which was sent via the Messenger app.

The company said the practice was "very common in tech" until firms started to put an end to it, with Apple having suspended access to Siri recordings for contractors following claims by a former worker that they "regularly" overheard confidential and intimate encounters, including drug deals and people having sex.

Amazon staff have also reportedly heard "distressing" recordings taken by Alexa voice assistants, and - like Apple and Google - have recently said the practice has been halted.

Facebook insisted that the audio snippets it used from Messenger were "totally de-identified" and "masked so as not to reveal anyone's identity".

The social media firm added that it "never listened to people's microphones without device permission and explicit activation by someone, and still don't".

Facebook, Google and Amazon are yet to be joined by Apple in offering users the ability to opt-out of its quality control scheme, whereby the company uses audio samples heard by its Siri assistant to try and make it more responsive.

Siri allows devices like the iPhone and Apple TV to be controlled with voice commands, and becomes active and starts to record upon hearing the phrase "Hey, Siri".

With no option to opt out of having Siri record audio samples, the only way to stop it from happening is by turning Siri off altogether in your device's settings.

The revelation that contractors have been hearing such recordings raised questions about Apple's commitment to privacy and its implementation of privacy designs.