Flutter: Move to a new screen without back

You need to use Navigator.pushReplacement when leaving the auth screen too. Not just when redirecting to login page.

You can use the pushAndRemoveUntil method:

Push the given route onto the navigator that most tightly encloses the given context, and then remove all the previous routes until the predicate returns true. To remove all the routes below the pushed route, use a [RoutePredicate] that always returns false (e.g. (Route<dynamic> route) => false).

Navigator.pushAndRemoveUntil(
  context,
  MaterialPageRoute(builder: (context) => MainPage()),
  (Route<dynamic> route) => false,
);

You need to use

Navigator
    .of(_context)
    .pushReplacement(new MaterialPageRoute(builder: (BuildContext context) => page));

Where _context is object of BuildContext And page is which page you directed to.