diff --git a/src/wp-includes/default-filters.php b/src/wp-includes/default-filters.php
index 4b6d9de25fa11..9c63a0744e069 100644
--- a/src/wp-includes/default-filters.php
+++ b/src/wp-includes/default-filters.php
@@ -627,6 +627,7 @@
 add_action( 'enqueue_block_editor_assets', 'wp_enqueue_block_editor_script_modules' );
 add_action( 'enqueue_block_editor_assets', 'wp_enqueue_global_styles_css_custom_properties' );
 add_action( 'enqueue_block_editor_assets', '_wp_enqueue_auto_register_blocks' );
+add_action( 'enqueue_block_editor_assets', 'wp_declare_classic_block_necessary' );
 add_action( 'wp_print_scripts', 'wp_just_in_time_script_localization' );
 add_filter( 'print_scripts_array', 'wp_prototype_before_jquery' );
 add_action( 'customize_controls_print_styles', 'wp_resource_hints', 1 );
diff --git a/src/wp-includes/script-loader.php b/src/wp-includes/script-loader.php
index 42d42b3f8781d..09ccfbfd14a0f 100644
--- a/src/wp-includes/script-loader.php
+++ b/src/wp-includes/script-loader.php
@@ -2647,6 +2647,35 @@ function wp_enqueue_global_styles() {
 	wp_add_global_styles_for_blocks();
 }
 
+/**
+ * Declares a flag that the Classic block is necessary for the current post.
+ *
+ * @global WP_Post $post Global post object.
+ *
+ * @since 7.1.0
+ */
+function wp_declare_classic_block_necessary() {
+	global $post;
+
+	/**
+	 * Filters whether the Classic block should be available in the inserter.
+	 *
+	 * Defaults to false. Use this filter to opt in (globally or per post).
+	 *
+	 * @param bool         $supports_inserter Whether the Classic block is available in the inserter.
+	 * @param WP_Post|null $post              The post being edited, or null if not in the post editor.
+	 */
+	if ( ! (bool) apply_filters( 'wp_classic_block_supports_inserter', false, $post ) ) {
+		return;
+	}
+
+	wp_add_inline_script(
+		'wp-block-library',
+		'window.__needsClassicBlock = true;',
+		'before'
+	);
+}
+
 /**
  * Checks if the editor scripts and styles for all registered block types
  * should be enqueued on the current screen.