diff --git a/extra/cssinliner-extra/CssInlinerExtension.php b/extra/cssinliner-extra/CssInlinerExtension.php
index 4d8b75a5623..679e9165716 100644
--- a/extra/cssinliner-extra/CssInlinerExtension.php
+++ b/extra/cssinliner-extra/CssInlinerExtension.php
@@ -20,17 +20,20 @@ class CssInlinerExtension extends AbstractExtension
     public function getFilters()
     {
         return [
-            new TwigFilter('inline_css', 'Twig\\Extra\\CssInliner\\twig_inline_css', ['is_safe' => ['all']]),
+            new TwigFilter('inline_css', [self::class, 'inlineCss'], ['is_safe' => ['all']]),
         ];
     }
-}
 
-function twig_inline_css(string $body, string ...$css): string
-{
-    static $inliner;
-    if (null === $inliner) {
-        $inliner = new CssToInlineStyles();
+    /**
+     * @internal
+     */
+    public static function inlineCss(string $body, string ...$css): string
+    {
+        static $inliner;
+        if (null === $inliner) {
+            $inliner = new CssToInlineStyles();
+        }
+    
+        return $inliner->convert($body, implode("\n", $css));
     }
-
-    return $inliner->convert($body, implode("\n", $css));
 }
diff --git a/extra/cssinliner-extra/Resources/functions.php b/extra/cssinliner-extra/Resources/functions.php
new file mode 100644
index 00000000000..c4b32db2d95
--- /dev/null
+++ b/extra/cssinliner-extra/Resources/functions.php
@@ -0,0 +1,23 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Twig\Extra\CssInliner;
+
+/**
+ * @internal
+ * @deprecated since Twig 3.9.0
+ */
+function twig_inline_css(string $body, string ...$css): string
+{
+    trigger_deprecation('twig/cssinliner-extra', '3.9.0', 'Using the internal "%s" function is deprecated.', __FUNCTION__);
+
+    return CssInlinerExtension::inlineCss($body, ...$css);
+}
diff --git a/extra/cssinliner-extra/composer.json b/extra/cssinliner-extra/composer.json
index 32be7b44de4..0a279c61323 100644
--- a/extra/cssinliner-extra/composer.json
+++ b/extra/cssinliner-extra/composer.json
@@ -16,6 +16,7 @@
     ],
     "require": {
         "php": ">=7.2.5",
+        "symfony/deprecation-contracts": "^2.5|^3",
         "tijsverkoyen/css-to-inline-styles": "^2.0",
         "twig/twig": "^3.0"
     },
@@ -23,6 +24,7 @@
         "symfony/phpunit-bridge": "^6.4|^7.0"
     },
     "autoload": {
+        "files": [ "Resources/functions.php" ],
         "psr-4" : { "Twig\\Extra\\CssInliner\\" : "" },
         "exclude-from-classmap": [
             "/Tests/"