Index: src/wp-admin/update-core.php
===================================================================
--- src/wp-admin/update-core.php	(revision 49555)
+++ src/wp-admin/update-core.php	(working copy)
@@ -322,6 +322,7 @@
 	$upgrade_minor = get_site_option( 'auto_update_core_minor', true );
 	$upgrade_major = get_site_option( 'auto_update_core_major', false );
 
+	$major_optin_disabled = '';
 	// WP_AUTO_UPDATE_CORE = true (all), 'beta', 'rc', 'minor', false.
 	if ( defined( 'WP_AUTO_UPDATE_CORE' ) ) {
 		if ( false === WP_AUTO_UPDATE_CORE ) {
@@ -343,6 +344,7 @@
 			$upgrade_minor = true;
 			$upgrade_major = false;
 		}
+		$major_optin_disabled = ' disabled';
 	}
 
 	/** This filter is documented in wp-admin/includes/class-core-upgrader.php */
@@ -352,6 +354,10 @@
 	/** This filter is documented in wp-admin/includes/class-core-upgrader.php */
 	$upgrade_major = apply_filters( 'allow_major_auto_core_updates', $upgrade_major );
 
+	$is_major_optin_disabled = apply_filters( 'allow_major_auto_core_updates', null );
+	if ( isset( $is_major_optin_disabled ) ) {
+		$major_optin_disabled = ' disabled';
+	}
 	$auto_update_settings = array(
 		'dev'   => $upgrade_dev,
 		'minor' => $upgrade_minor,
@@ -361,21 +367,22 @@
 	<form method="post" action="<?php echo esc_url( $_SERVER['REQUEST_URI'] ); ?>" name="core-auto-updates" class="form-core-auto-updates">
 		<?php wp_nonce_field( 'core-auto-updates-nonce', 'set_core_auto_updates_settings' ); ?>
 		<h2><?php _e( 'Auto-update settings' ); ?></h2>
-		<p>
-			<?php
-			if ( $auto_update_settings['major'] ) {
-				$wp_version = get_bloginfo( 'version' );
-				$updates    = get_core_updates();
-				if ( isset( $updates[0]->version ) && version_compare( $updates[0]->version, $wp_version, '>' ) ) {
-					echo wp_get_auto_update_message();
-				}
+		<?php
+		if ( $auto_update_settings['major'] ) {
+			$wp_version = get_bloginfo( 'version' );
+			$updates    = get_core_updates();
+			if ( isset( $updates[0]->version ) && version_compare( $updates[0]->version, $wp_version, '>' ) ) {
+				echo '<p>'.wp_get_auto_update_message().'</p>';
 			}
-			?>
-		</p>
+		}
+		?>
 		<p>
-			<input type="checkbox" name="core-auto-updates-major" id="core-auto-updates-major" value="1" <?php checked( $auto_update_settings['major'], 1 ); ?> />
+			<input type="checkbox" name="core-auto-updates-major" id="core-auto-updates-major" value="1"<?php checked( $auto_update_settings['major'], 1 ); ?><?php echo $major_optin_disabled; ?> />
 			<label for="core-auto-updates-major">
 				<?php _e( 'Automatically keep this site up-to-date with regular feature updates.' ); ?>
+				<?php if ( ! empty( $major_optin_disabled ) ) : ?>
+					<em><?php _e( 'This setting has been disabled using a constant or a filter.' ); ?></em>
+				<?php endif; ?>
 			</label>
 		</p>
 		<?php