diff --git a/src/Composer/Command/InstallCommand.php b/src/Composer/Command/InstallCommand.php
index 32fb1bdc6..231321406 100644
--- a/src/Composer/Command/InstallCommand.php
+++ b/src/Composer/Command/InstallCommand.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Command;
 
+use Composer\Composer;
 use Composer\Installer;
 use Composer\Plugin\CommandEvent;
 use Composer\Plugin\PluginEvents;
@@ -85,6 +86,8 @@ EOT
             $io->writeError('<warning>You are using the deprecated option "dev". Dev packages are installed by default now.</warning>');
         }
 
+        $io->writeError('<warning>You are using Composer 1 which is deprecated. You should upgrade to Composer 2, see https://blog.packagist.com/deprecating-composer-1-support/</warning>');
+
         $composer = $this->getComposer(true, $input->getOption('no-plugins'));
         $composer->getDownloadManager()->setOutputProgress(!$input->getOption('no-progress'));