The UK's Information Commissioner (ICO) is looking into WhatsApp's decision to share more data with parent company Facebook.
The data about WhatsApp users will be used to help targeted advertising seen by people on Facebook.
It will share phone numbers and the details of the last time that users signed on to WhatsApp, with Facebook.
The ICO said because the changes would affect a lot of people it wanted more details about what was being shared.
Information Commissioner Elizabeth Denham said in a statement that users' opinions about the change was likely to be split.
"Some might consider it'll give them a better service, others may be concerned by the lack of control," she said.
"Our role is to pull back the curtain on things like this, ensuring that companies are being transparent with the public about how their personal data is being shared and protecting consumers by making sure the law is being followed."
While organisations did not need to get prior permission from the ICO to change how they handle personal data, any change had to remain within data protection laws, she added.
Any organisation that breaks the Data Protection Act can be fined up to £500,000 by the ICO.
WhatsApp revealed the changes in a blogpost and said sharing more data would let it provide "more relevant" suggestions about who users should connect with.
Spam and abuse would also be better tackled thanks to the data-sharing deal, it added. The messaging firm also provided instructions for people who did not want their data to be shared.
One analyst said some users might see the change of policy as a "betrayal" because the company had previously pledged to remain independent of Facebook.