Data privacy authorities in Germany on Tuesday ordered Facebook to delete data it has received on Whatsapp users in Germany.
Facebook, which acquired Whatsapp in 2014 for 22 billion dollars, is ordered to delete the Whatsapp user data immediately or face fines from German authorities.
In August the mobile messaging app updated its policy to allow owner Facebook to access to more data, including users' phone numbers, in what was Whatsapp's first policy update since it was acquired.
However Data Protection Commissioner Johannes Caspar said Germany's 35 million Whatsapp users should be able to decide for themselves if they want their Whatsapp and Facebook accounts to be connected.
"To do this, Facebook needs to ask their permission. This was not the case," Caspar said.
The company had said it would not "sell, share or give" users' phone numbers to advertisers, but that Facebook would use them to provide "better friend suggestions" and "more relevant ads."
The social media giant responded by saying it "follows EU data protection laws," and is open to solving the concerns raised by German authorities.
Facebook has long said that since its European operation is based in Dublin, only Irish data privacy laws apply.