WhatsApp, which counts more than a billion users globally, said on Thursday it would start sharing users' phone numbers with Facebook, helping it to target advertisements and friend recommendations across the social media network.
It said WhatsApp users could choose not to share their account information with Facebook.
"What is at stake is the control of individual users over their own data when they are combined by major Internet players."
The G29 is made up of representatives of the data protection authorities of each of Europe's 28-member states, the European Data Protection Supervisor and the European Commission.
In response, Britain's data privacy regulator, the Information Commissioner's Office (ICO), said it was monitoring the changes to ensure that the new policy stays within data protection laws (http://reut.rs/2bwpIdX).
The French data protection authority is studying Facebook's response earlier this month to issues the regulator has raised over the company's handling of customer data for targeting advertising to the social network's members.
CNIL has yet to receive any specific complaints from citizens to the latest changes to WhatsApp privacy policies, it said.
By Monday several European media including France Info and Switzerland's Le Temps warned readers about the policy change and gave them tips on how to avoid letting WhatsApp handing over their phone number to Facebook.
(Reporting by Eric Auchard and Astrid Wendlandt; editing by Susan Thomas)