Starting next year, Apple will remove apps from its App Store that track users without first receiving permission, a move that promises to bolster iPhone user privacy but will likely shake the app advertising industry.

To target ads and measure how effective they are, app developers and other industry players currently often use an Identifier for Advertisers (IDFA), or a string of letters and numbers that’s different on every Apple device.

In an update to the iPhone operating system expected “early next year,” app makers will be forced to ask permission to access a user’s IDFA through a pop-up. A significant portion of users will likely decide to opt-out, reducing the effectiveness and profitability of targeted ads. The change takes a privacy option that was previously buried in settings and puts it front and center when users open each app.

On Tuesday, Apple’s senior vice president of software engineering Craig Federighi said apps that don’t comply with the new requirements, which Apple calls App Tracking Transparency (ATT), can be removed from the App Store, which is the only way to install software on an iPhone.

The move pits app developers who make money through targeted ads against Apple, which has increasingly built privacy features into its products as a way to differentiate them from competitors. Critics include Facebook, which said that the change could reduce revenue by 50% in one of its advertising businesses.

″Some in the ad industry are lobbying against these efforts—claiming that ATT will dramatically hurt ad-supported businesses—but we expect that the industry will adapt as it did when we introduced intelligent tracking prevention, providing effective advertising without invasive tracking,” Federighi said in a speech at a European privacy conference.

CNBC