Many WhatsApp users could find the messaging app is no longer working on their smartphones as of Monday.
In February 2016, the -owned service said it would end support for users on the following old operating systems:
WhatsApp users on OS, BlackBerry 10, Nokia S40 and Nokia Symbian S60 will have until June 30, 2017.
The popular messaging app, which now has over 1 billion users the company claims, said that it decided to make this decision in order to continue introducing more advanced features that old operating systems can't handle.
"While these mobile devices have been an important part of our story, they don't offer the kind of capabilities we need to expand our app's features in the future," WhatsApp said in a blog post in February.
The company urged people to upgrade their devices to newer versions of Android, iPhone or Windows Phone.
WhatsApp said it is putting "an even greater emphasis on security features" which would require a more advanced operating system. The app recently launched a video calling feature and is likely to be rolling out more this year.