diff --git a/src/Composer/Plugin/PluginManager.php b/src/Composer/Plugin/PluginManager.php
index c0c86b7b6..29cca6a19 100644
--- a/src/Composer/Plugin/PluginManager.php
+++ b/src/Composer/Plugin/PluginManager.php
@@ -276,7 +276,7 @@ class PluginManager
                 if ($separatorPos) {
                     $className = substr($class, $separatorPos + 1);
                 }
-                $code = Preg::replace('{^((?:final\s+)?(?:\s*))class\s+('.preg_quote($className).')}mi', '$1class $2_composer_tmp'.self::$classCounter, $code, 1);
+                $code = Preg::replace('{^((?:(?:final|readonly)\s+)*(?:\s*))class\s+('.preg_quote($className).')}mi', '$1class $2_composer_tmp'.self::$classCounter, $code, 1);
                 $code = strtr($code, array(
                     '__FILE__' => var_export($path, true),
                     '__DIR__' => var_export(dirname($path), true),