Google is continuing to tighten the rules when it comes to ads that appear in Android apps and is warning developers to be more clear about their programs' data collection behavior.

The tech giant's Safe Browsing team has expanded its Unwanted Software Policy to address further "unwanted and harmful behaviors on Android." Apps that handle personal user data such as phone numbers and email addresses, or data about the device (such as IMEI number), must prompt users for permission to collect the information. The app's privacy policy must also be displayed within the application.

The revised rules also state that if an app collects and transmits personal data that is unrelated to its functions, then it must highlight this fact before transmission and seek consent from the user first. This covers app functions such as crash reports, where a list of installed apps on a device cannot be sent without permission.

If developers don't comply within 60 days, Google will warn users via Google Play Protect or on websites that lead to these apps.

Google has been cracking down on certain ads within apps for a long time. Those that are considered deceptive, disruptive, inappropriate or interfere with applications or device functionality are regularly removed from the store. Last week, an update to the Google Play Policy Center revealed that apps which introduce ads to the lock screen were being banned from the Play Store, the only exception being those apps that function exclusively as lock screens.

Late last month, researchers found that three-quarters of the 300+ Android apps they analyzed, which included Uber, Spotify, and Tinder, contained embedded third-party trackers.