diff --git a/phpBB/includes/startup.php b/phpBB/includes/startup.php
index 9c4e1374ba..45eaff6fc7 100644
--- a/phpBB/includes/startup.php
+++ b/phpBB/includes/startup.php
@@ -149,22 +149,32 @@ if (function_exists('date_default_timezone_set') && function_exists('date_defaul
 
 // Autoloading of dependencies.
 // Three options are supported:
-// 1. Specify PHPBB_AUTOLOAD=/path/to/autoload.php in the environment.
-//    This is useful for running CLI scripts and tests.
+// 1. If dependencies are installed with Composer, Composer will create a
+//    vendor/.composer/autoload.php. If this file exists it will be
+//    automatically used by phpBB. This is the default mode that phpBB
+//    will use when shipped.
+// 2. To disable composer autoloading, PHPBB_NO_AUTOLOAD can be specified.
+// 	  Additionally specify PHPBB_AUTOLOAD=/path/to/autoload.php in the
+//    environment. This is useful for running CLI scripts and tests.
 //    /path/to/autoload.php should define and register class loaders
 //    for all of phpBB's dependencies.
-// 2. If dependencies are installed with Composer, Composer will create a
-//    vendor/.composer/autoload.php. If this file exists it will be
-//    automatically used by phpBB.
-// 3. Failing that phpBB assumes that autoloading has been set up in
-//    some other way. This might be useful in cases when phpBB is integrated
-//    into a larger program.
-if (getenv('PHPBB_AUTOLOAD'))
+// 3. You can also set PHPBB_NO_AUTOLOAD without setting PHPBB_AUTOLOAD.
+//    In this case autoloading needs to be defined before running any phpBB
+//    script. This might be useful in cases when phpBB is integrated into a
+//    larger program.
+if (getenv('PHPBB_NO_AUTOLOAD'))
 {
-	require(getenv('PHPBB_AUTOLOAD'));
+	if (getenv('PHPBB_AUTOLOAD'))
+	{
+		require(getenv('PHPBB_AUTOLOAD'));
+	}
 }
-else if (file_exists($phpbb_root_path . 'vendor/.composer/autoload.php'))
+else
 {
+	if (!file_exists($phpbb_root_path . 'vendor/.composer/autoload.php'))
+	{
+		trigger_error('You have not set up composer dependencies. See http://getcomposer.org/.', E_USER_ERROR);
+	}
 	require($phpbb_root_path . 'vendor/.composer/autoload.php');
 }