A new WhatsApp update will prevent the popular messaging app from working on millions of phones from 1 January 2021.

Older Android and iPhone devices will no longer support the latest version of the app, forcing owners to either update their mobile operating system or buy a new smartphone, Yahoo News reports.

Any iPhone not running iOS 9 or newer, or any phone not running at least Android 4.0.3 – also known as Ice Cream Sandwich – will no longer be able to open or operate the Facebook-owned app.

This means all iPhone models up to the iPhone 4 will be unable to support WhatsApp, as these phones are not capable of updating to iOS 9.

The iPhone 4S, iPhone 5, iPhone 5S, iPhone 6 and iPhone 6S were all released before iOS 9, however owners can still update to the operating system if they are yet to do so.

Android phones that will lose WhatsApp support include the Samsung Galaxy S2, HTC Desire and LG Optimus Black.

Phone owners can see what software their device is using by checking the settings section. For iPhone users, this can be found under Settings > General > Software update.

Most of the 2 billion WhatsApp users around the world will not be hit by the change, though the small fraction that are will likely comprise several million people worldwide.

A similar update at the start of 2020 saw WhatsApp make the “tough decision” of ending support for even older phones.

Phones set to lose WhatsApp support on 1 January 2021:

Apple iPhone 1-4

Samsung Galaxy S2

HTC Desire

LG Optimus Black

Motorola Droid Razr

Any Android released before 2010

Phones that will need to update to at least iOS 9 or Android OS 4.0.3:

Apple iPhone 4S

iPhone 5

iPhone 5S

iPhone 6

iPhone 6S

Samsung Galaxy S3 and newer

Samsung Galaxy Note

HTC Sensation

HTC Thunderbolt

LG Lucid

Motorola Droid 4

Sony Xperia Pro and newer

The messaging giant will also be updating its Terms of Service next year, forcing users to agree to new privacy rules in order to keep using the app.