diff --git a/CHANGELOG b/CHANGELOG
index a0b9de2b713..c6b35ba5b56 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,6 +1,7 @@
-# 3.7.2 (2023-XX-XX)
+# 3.8.0 (2023-XX-XX)
 
- * n/a
+ * Deprecate all internal extension functions in favor of methods on the extension classes
+ * Mark all extension functions as @internal
 
 # 3.7.1 (2023-08-28)
 
diff --git a/composer.json b/composer.json
index 04dc93ffccf..7d35967b2b2 100644
--- a/composer.json
+++ b/composer.json
@@ -25,6 +25,7 @@
     ],
     "require": {
         "php": ">=7.2.5",
+        "symfony/deprecation-contracts": "^2.5|^3",
         "symfony/polyfill-mbstring": "^1.3",
         "symfony/polyfill-ctype": "^1.8"
     },
diff --git a/extra/html-extra/HtmlExtension.php b/extra/html-extra/HtmlExtension.php
index ed740b47187..9704ebb6efb 100644
--- a/extra/html-extra/HtmlExtension.php
+++ b/extra/html-extra/HtmlExtension.php
@@ -9,8 +9,10 @@
  * file that was distributed with this source code.
  */
 
-namespace Twig\Extra\Html {
+namespace Twig\Extra\Html;
+
 use Symfony\Component\Mime\MimeTypes;
+use Twig\Error\RuntimeError;
 use Twig\Extension\AbstractExtension;
 use Twig\TwigFilter;
 use Twig\TwigFunction;
@@ -34,7 +36,7 @@ public function getFilters(): array
     public function getFunctions(): array
     {
         return [
-            new TwigFunction('html_classes', 'twig_html_classes'),
+            new TwigFunction('html_classes', [get_class($this), 'htmlClasses']),
         ];
     }
 
@@ -45,6 +47,8 @@ public function getFunctions(): array
      * be done before calling this filter.
      *
      * @return string The generated data URI
+     *
+     * @internal
      */
     public function dataUri(string $data, string $mime = null, array $parameters = []): string
     {
@@ -79,33 +83,31 @@ public function dataUri(string $data, string $mime = null, array $parameters = [
 
         return $repr;
     }
-}
-}
-
-namespace {
-use Twig\Error\RuntimeError;
 
-function twig_html_classes(...$args): string
-{
-    $classes = [];
-    foreach ($args as $i => $arg) {
-        if (\is_string($arg)) {
-            $classes[] = $arg;
-        } elseif (\is_array($arg)) {
-            foreach ($arg as $class => $condition) {
-                if (!\is_string($class)) {
-                    throw new RuntimeError(sprintf('The html_classes function argument %d (key %d) should be a string, got "%s".', $i, $class, \gettype($class)));
-                }
-                if (!$condition) {
-                    continue;
+    /**
+     * @internal
+     */
+    public static function htmlClasses(...$args): string
+    {
+        $classes = [];
+        foreach ($args as $i => $arg) {
+            if (\is_string($arg)) {
+                $classes[] = $arg;
+            } elseif (\is_array($arg)) {
+                foreach ($arg as $class => $condition) {
+                    if (!\is_string($class)) {
+                        throw new RuntimeError(sprintf('The html_classes function argument %d (key %d) should be a string, got "%s".', $i, $class, \gettype($class)));
+                    }
+                    if (!$condition) {
+                        continue;
+                    }
+                    $classes[] = $class;
                 }
-                $classes[] = $class;
+            } else {
+                throw new RuntimeError(sprintf('The html_classes function argument %d should be either a string or an array, got "%s".', $i, \gettype($arg)));
             }
-        } else {
-            throw new RuntimeError(sprintf('The html_classes function argument %d should be either a string or an array, got "%s".', $i, \gettype($arg)));
         }
-    }
 
-    return implode(' ', array_unique($classes));
-}
+        return implode(' ', array_unique($classes));
+    }
 }
diff --git a/extra/html-extra/Resources/functions.php b/extra/html-extra/Resources/functions.php
new file mode 100644
index 00000000000..619789b6378
--- /dev/null
+++ b/extra/html-extra/Resources/functions.php
@@ -0,0 +1,22 @@
+<?php
+
+/*
+ * This file is part of Twig.
+ *
+ * (c) Fabien Potencier
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Twig\Extra\Html\HtmlExtension;
+
+/**
+ * @internal
+ */
+function twig_html_classes(...$args): string
+{
+    trigger_deprecation('twig/html-extra', '3.8.0', 'Using the internal "%s" function is deprecated.', __FUNCTION__);
+
+    return HtmlExtension::htmlClasses(...$args);
+}
diff --git a/extra/html-extra/composer.json b/extra/html-extra/composer.json
index 98670abfacc..199ff23048f 100644
--- a/extra/html-extra/composer.json
+++ b/extra/html-extra/composer.json
@@ -16,6 +16,7 @@
     ],
     "require": {
         "php": ">=7.1.3",
+        "symfony/deprecation-contracts": "^2.5|^3",
         "symfony/mime": "^5.4|^6.0|^7.0",
         "twig/twig": "^2.7|^3.0"
     },
@@ -23,6 +24,7 @@
         "symfony/phpunit-bridge": "^5.4|^6.3|^7.0"
     },
     "autoload": {
+        "files": [ "Resources/functions.php" ],
         "psr-4" : { "Twig\\Extra\\Html\\" : "" },
         "exclude-from-classmap": [
             "/Tests/"