diff --git a/doc/06-config.md b/doc/06-config.md
index 636cf955508c..d89aa062d913 100644
--- a/doc/06-config.md
+++ b/doc/06-config.md
@@ -396,7 +396,7 @@ in the Composer home, cache, and data directories.
 ## lock
 
 Defaults to `true`. If set to `false`, Composer will not create a `composer.lock`
-file.
+file and will ignore it if one is present.
 
 ## platform-check
 
diff --git a/src/Composer/Factory.php b/src/Composer/Factory.php
index ee0b07f3e3bb..9288da19b914 100644
--- a/src/Composer/Factory.php
+++ b/src/Composer/Factory.php
@@ -442,8 +442,11 @@ public function createComposer(IOInterface $io, $localConfig = null, $disablePlu
         // init locker if possible
         if ($fullLoad && isset($composerFile)) {
             $lockFile = self::getLockFile($composerFile);
+            if (!$config->get('lock') && file_exists($lockFile)) {
+                $io->writeError('<warning>'.$lockFile.' is present but ignored as the "lock" config option is disabled.</warning>');
+            }
 
-            $locker = new Package\Locker($io, new JsonFile($lockFile, null, $io), $im, file_get_contents($composerFile), $process);
+            $locker = new Package\Locker($io, new JsonFile($config->get('lock') ? $lockFile : Platform::getDevNull(), null, $io), $im, file_get_contents($composerFile), $process);
             $composer->setLocker($locker);
         }
 
diff --git a/src/Composer/Util/Platform.php b/src/Composer/Util/Platform.php
index 756c115063ac..163bf36d4aed 100644
--- a/src/Composer/Util/Platform.php
+++ b/src/Composer/Util/Platform.php
@@ -249,4 +249,13 @@ private static function isVirtualBoxGuest()
 
         return self::$isVirtualBoxGuest;
     }
+
+    public static function getDevNull()
+    {
+        if (self::isWindows()) {
+            return 'NUL';
+        }
+
+        return '/dev/null';
+    }
 }