When signing in to a Google service (like Gmail), after you provide your username and password, your mobile browser receives an authentication token. This allows you to access Gmail, Google Calendar, and other Google services without having to sign in everytime. Authentication tokens are typically stored in the form of a cookie. A few older mobile browsers don't support cookies, and so we temporarily retained a legacy mechanism for authentication.
How do I know if my mobile browser is affected?
If you're using an older mobile browser that doesn't support authentication cookies, you'll see an alert when you try to sign in to your Google Account or G Suite account.
If you have an unsupported mobile browser, we suggest you check the mobile browser requirements for Gmail to find a compatible device. We are working to provide the best experience with Google products using the technologies provided by modern mobile browsers.
Check with your mobile phone provider to see if the browser on your mobile device can handle cookies. If you disable cookies on your mobile browser, it won't be able to store the authentication cookie, and you won't be able to sign in to Google. For more information, see Browser's cookie functionality turned off.