By Stephen Nellis

(Reuters) – Apple Inc said on Thursday that new privacy pop-up notifications will start appearing on most iPhones as soon as “early spring,” a requirement that major digital advertising firms such as Facebook Inc have warned will harm their businesses.

The one-time notices will require an app developer to ask a user’s permission before the app tracks their activities “across other companies’ apps and websites.” Digital advertising experts believe that the warning will cause many users to decline permission.

Apple announced the move last June but said in September that it would delay the change to give digital advertisers more time to adjust.

Facebook said in December that it plans to show the pop-up notification because it did not want Apple iPhone users to lose access to its apps.

On an earnings call on Wednesday, Facebook executives told investors that the change could start hurting the company’s revenues in the first quarter, with Chief Executive Mark Zuckerberg accusing Apple of having “every incentive to use their dominant platform position to interfere with how our apps and other apps work.”

Apple on Wednesday disclosed that it has an active installed base of 1.65 billion devices, more than 1 billion of which are iPhones, with 620 million paying subscribers on its devices.

For its part, Alphabet Inc’s Google on Tuesday said that it would cease practices, including the use of an Apple-supplied tracking identifier, that would require it to show the warning, thus avoiding it.

Apple has said that is offering alternative technology free of charge that will help advertisers attribute paid clicks and taps without engaging in what Apple deems tracking.

Google said on Tuesday that is proactively working with Apple to improve the alternative offering.

Apple said on Wednesday that it will roll out new tools such as a way to attribute clicks made on video advertisements.

(Reporting by Stephen Nellis, Paresh Dave and Katie Paul in San Francisco; Editing by Sonya Hepinstall)