Facebook is the latest company to confirm it has used third-party workers to do such work, following Google, Apple, Microsoft and Amazon.

The practice had been halted "more than a week ago", Facebook said.

Workers were given audio of people's conversations but were not told how it was obtained, according to Bloomberg, which was first to report the news.

Facebook said the recordings were being transcribed manually so that artificial intelligence systems used to automatically transcribe conversations could be improved.

It added this had been done only when users had opted in to transcription services and given permission for microphone access.

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

Facebook's lead supervisory authority on data protection within the EU is Ireland's Data Protection Commissioner.

"We are now seeking detailed information from Facebook on the processing in question and how Facebook believes that such processing of data is compliant with their GDPR [General Data Protection Regulation] obligations," a spokeswoman told BBC News.

Earlier this month, Apple and Google said they had paused the practice of getting workers to listen to voice recordings of users so they could be transcribed.

Microsoft also confirmed voice recordings of users of Skype's automatic translation service were also analysed by humans.

The data regulator in Luxembourg is also currently in discussions with Amazon about its similar use of workers to check some Alexa voice recordings.