Facebook does not have "valid consent" from Whatsapp users to use their data for targeted adverts, the Information Commissioner's Office (ICO) has said.
In August, WhatsApp announced it was going to share users' information with parent company Facebook.
After a probe by the ICO, Facebook has agreed to hold off using data from UK users.
But Whatsapp is still planning to share information with Facebook.
Information Commissioner Elizabeth Denham said: "I had concerns that consumers weren't being properly protected, and it's fair to say the enquiries my team have made haven't changed that view.
"I don't think users have been given enough information about what Facebook plans to do with their information, and I don't think WhatsApp has got valid consent from users to share the information.
"I also believe users should be given ongoing control over how their information is used, not just a 30-day window."
Whatsapp said in August that sharing information with Facebook would mean "better friend suggestions" and "more relevant ads".