diff --git a/UPGRADE-5.1.md b/UPGRADE-5.1.md
index ea110ecc66f8b..afa2f217ef430 100644
--- a/UPGRADE-5.1.md
+++ b/UPGRADE-5.1.md
@@ -1,6 +1,11 @@
 UPGRADE FROM 5.0 to 5.1
 =======================
 
+EventDispatcher
+---------------
+
+ * Deprecated `LegacyEventDispatcherProxy`. Use the event dispatcher without the proxy.
+
 FrameworkBundle
 ---------------
 
diff --git a/UPGRADE-6.0.md b/UPGRADE-6.0.md
index 1b97c43d9b2a9..31b749f65db4e 100644
--- a/UPGRADE-6.0.md
+++ b/UPGRADE-6.0.md
@@ -1,6 +1,11 @@
 UPGRADE FROM 5.x to 6.0
 =======================
 
+EventDispatcher
+---------------
+
+ * Removed `LegacyEventDispatcherProxy`. Use the event dispatcher without the proxy.
+
 FrameworkBundle
 ---------------
 
diff --git a/src/Symfony/Component/EventDispatcher/CHANGELOG.md b/src/Symfony/Component/EventDispatcher/CHANGELOG.md
index ce30074f8ae30..f4bddd3b54160 100644
--- a/src/Symfony/Component/EventDispatcher/CHANGELOG.md
+++ b/src/Symfony/Component/EventDispatcher/CHANGELOG.md
@@ -1,6 +1,11 @@
 CHANGELOG
 =========
 
+5.1.0
+-----
+
+ * The `LegacyEventDispatcherProxy` class has been deprecated.
+
 5.0.0
 -----
 
diff --git a/src/Symfony/Component/EventDispatcher/LegacyEventDispatcherProxy.php b/src/Symfony/Component/EventDispatcher/LegacyEventDispatcherProxy.php
index a44b766cc1ecc..c01e3f647c250 100644
--- a/src/Symfony/Component/EventDispatcher/LegacyEventDispatcherProxy.php
+++ b/src/Symfony/Component/EventDispatcher/LegacyEventDispatcherProxy.php
@@ -13,12 +13,14 @@
 
 use Symfony\Contracts\EventDispatcher\EventDispatcherInterface;
 
+@trigger_error(sprintf('%s is deprecated since Symfony 5.1, use the event dispatcher without the proxy.', LegacyEventDispatcherProxy::class), E_USER_DEPRECATED);
+
 /**
  * A helper class to provide BC/FC with the legacy signature of EventDispatcherInterface::dispatch().
  *
- * This class should be deprecated in Symfony 5.1
- *
  * @author Nicolas Grekas <p@tchwork.com>
+ *
+ * @deprecated since Symfony 5.1.
  */
 final class LegacyEventDispatcherProxy
 {
diff --git a/src/Symfony/Component/Mailer/Mailer.php b/src/Symfony/Component/Mailer/Mailer.php
index 260989e72166a..a7fb0daa84da3 100644
--- a/src/Symfony/Component/Mailer/Mailer.php
+++ b/src/Symfony/Component/Mailer/Mailer.php
@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Mailer;
 
+use Symfony\Component\EventDispatcher\Event;
 use Symfony\Component\EventDispatcher\LegacyEventDispatcherProxy;
 use Symfony\Component\Mailer\Event\MessageEvent;
 use Symfony\Component\Mailer\Messenger\SendEmailMessage;
@@ -32,7 +33,7 @@ public function __construct(TransportInterface $transport, MessageBusInterface $
     {
         $this->transport = $transport;
         $this->bus = $bus;
-        $this->dispatcher = LegacyEventDispatcherProxy::decorate($dispatcher);
+        $this->dispatcher = class_exists(Event::class) ? LegacyEventDispatcherProxy::decorate($dispatcher) : $dispatcher;
     }
 
     public function send(RawMessage $message, Envelope $envelope = null): void
diff --git a/src/Symfony/Component/Mailer/Transport/AbstractTransport.php b/src/Symfony/Component/Mailer/Transport/AbstractTransport.php
index 1bc3fa12a5616..cf21c724b3137 100644
--- a/src/Symfony/Component/Mailer/Transport/AbstractTransport.php
+++ b/src/Symfony/Component/Mailer/Transport/AbstractTransport.php
@@ -13,6 +13,7 @@
 
 use Psr\Log\LoggerInterface;
 use Psr\Log\NullLogger;
+use Symfony\Component\EventDispatcher\Event;
 use Symfony\Component\EventDispatcher\LegacyEventDispatcherProxy;
 use Symfony\Component\Mailer\Envelope;
 use Symfony\Component\Mailer\Event\MessageEvent;
@@ -33,7 +34,7 @@ abstract class AbstractTransport implements TransportInterface
 
     public function __construct(EventDispatcherInterface $dispatcher = null, LoggerInterface $logger = null)
     {
-        $this->dispatcher = LegacyEventDispatcherProxy::decorate($dispatcher);
+        $this->dispatcher = class_exists(Event::class) ? LegacyEventDispatcherProxy::decorate($dispatcher) : $dispatcher;
         $this->logger = $logger ?: new NullLogger();
     }
 
diff --git a/src/Symfony/Component/Messenger/Middleware/SendMessageMiddleware.php b/src/Symfony/Component/Messenger/Middleware/SendMessageMiddleware.php
index 149c0c2415f55..213e797da75c5 100644
--- a/src/Symfony/Component/Messenger/Middleware/SendMessageMiddleware.php
+++ b/src/Symfony/Component/Messenger/Middleware/SendMessageMiddleware.php
@@ -13,6 +13,7 @@
 
 use Psr\Log\LoggerAwareTrait;
 use Psr\Log\NullLogger;
+use Symfony\Component\EventDispatcher\Event;
 use Symfony\Component\EventDispatcher\LegacyEventDispatcherProxy;
 use Symfony\Component\Messenger\Envelope;
 use Symfony\Component\Messenger\Event\SendMessageToTransportsEvent;
@@ -35,13 +36,7 @@ class SendMessageMiddleware implements MiddlewareInterface
     public function __construct(SendersLocatorInterface $sendersLocator, EventDispatcherInterface $eventDispatcher = null)
     {
         $this->sendersLocator = $sendersLocator;
-
-        if (null !== $eventDispatcher && class_exists(LegacyEventDispatcherProxy::class)) {
-            $this->eventDispatcher = LegacyEventDispatcherProxy::decorate($eventDispatcher);
-        } else {
-            $this->eventDispatcher = $eventDispatcher;
-        }
-
+        $this->eventDispatcher = class_exists(Event::class) ? LegacyEventDispatcherProxy::decorate($eventDispatcher) : $eventDispatcher;
         $this->logger = new NullLogger();
     }
 
diff --git a/src/Symfony/Component/Messenger/Worker.php b/src/Symfony/Component/Messenger/Worker.php
index 9ac7ebe697ce8..a1cca6b8023cc 100644
--- a/src/Symfony/Component/Messenger/Worker.php
+++ b/src/Symfony/Component/Messenger/Worker.php
@@ -12,6 +12,7 @@
 namespace Symfony\Component\Messenger;
 
 use Psr\Log\LoggerInterface;
+use Symfony\Component\EventDispatcher\Event;
 use Symfony\Component\EventDispatcher\LegacyEventDispatcherProxy;
 use Symfony\Component\Messenger\Event\WorkerMessageFailedEvent;
 use Symfony\Component\Messenger\Event\WorkerMessageHandledEvent;
@@ -48,12 +49,7 @@ public function __construct(array $receivers, MessageBusInterface $bus, EventDis
         $this->receivers = $receivers;
         $this->bus = $bus;
         $this->logger = $logger;
-
-        if (null !== $eventDispatcher && class_exists(LegacyEventDispatcherProxy::class)) {
-            $this->eventDispatcher = LegacyEventDispatcherProxy::decorate($eventDispatcher);
-        } else {
-            $this->eventDispatcher = $eventDispatcher;
-        }
+        $this->eventDispatcher = class_exists(Event::class) ? LegacyEventDispatcherProxy::decorate($eventDispatcher) : $eventDispatcher;
     }
 
     /**
diff --git a/src/Symfony/Component/Notifier/Chatter.php b/src/Symfony/Component/Notifier/Chatter.php
index 63d75f25098b9..bbaf2841e8cfd 100644
--- a/src/Symfony/Component/Notifier/Chatter.php
+++ b/src/Symfony/Component/Notifier/Chatter.php
@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Notifier;
 
+use Symfony\Component\EventDispatcher\Event;
 use Symfony\Component\EventDispatcher\LegacyEventDispatcherProxy;
 use Symfony\Component\Messenger\MessageBusInterface;
 use Symfony\Component\Notifier\Event\MessageEvent;
@@ -33,7 +34,7 @@ public function __construct(TransportInterface $transport, MessageBusInterface $
     {
         $this->transport = $transport;
         $this->bus = $bus;
-        $this->dispatcher = LegacyEventDispatcherProxy::decorate($dispatcher);
+        $this->dispatcher = class_exists(Event::class) ? LegacyEventDispatcherProxy::decorate($dispatcher) : $dispatcher;
     }
 
     public function __toString(): string
diff --git a/src/Symfony/Component/Notifier/Texter.php b/src/Symfony/Component/Notifier/Texter.php
index 957c5b30ce9e4..b5943380170e0 100644
--- a/src/Symfony/Component/Notifier/Texter.php
+++ b/src/Symfony/Component/Notifier/Texter.php
@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Notifier;
 
+use Symfony\Component\EventDispatcher\Event;
 use Symfony\Component\EventDispatcher\LegacyEventDispatcherProxy;
 use Symfony\Component\Messenger\MessageBusInterface;
 use Symfony\Component\Notifier\Event\MessageEvent;
@@ -33,7 +34,7 @@ public function __construct(TransportInterface $transport, MessageBusInterface $
     {
         $this->transport = $transport;
         $this->bus = $bus;
-        $this->dispatcher = LegacyEventDispatcherProxy::decorate($dispatcher);
+        $this->dispatcher = class_exists(Event::class) ? LegacyEventDispatcherProxy::decorate($dispatcher) : $dispatcher;
     }
 
     public function __toString(): string
diff --git a/src/Symfony/Component/Notifier/Transport/AbstractTransport.php b/src/Symfony/Component/Notifier/Transport/AbstractTransport.php
index 1c22359feb219..b6f28660225ed 100644
--- a/src/Symfony/Component/Notifier/Transport/AbstractTransport.php
+++ b/src/Symfony/Component/Notifier/Transport/AbstractTransport.php
@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Notifier\Transport;
 
+use Symfony\Component\EventDispatcher\Event;
 use Symfony\Component\EventDispatcher\LegacyEventDispatcherProxy;
 use Symfony\Component\HttpClient\HttpClient;
 use Symfony\Component\Notifier\Event\MessageEvent;
@@ -45,7 +46,7 @@ public function __construct(HttpClientInterface $client = null, EventDispatcherI
             $this->client = HttpClient::create();
         }
 
-        $this->dispatcher = LegacyEventDispatcherProxy::decorate($dispatcher);
+        $this->dispatcher = class_exists(Event::class) ? LegacyEventDispatcherProxy::decorate($dispatcher) : $dispatcher;
     }
 
     /**
diff --git a/src/Symfony/Component/Notifier/Transport/AbstractTransportFactory.php b/src/Symfony/Component/Notifier/Transport/AbstractTransportFactory.php
index be92b3c57883b..1edbbb07138aa 100644
--- a/src/Symfony/Component/Notifier/Transport/AbstractTransportFactory.php
+++ b/src/Symfony/Component/Notifier/Transport/AbstractTransportFactory.php
@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Notifier\Transport;
 
+use Symfony\Component\EventDispatcher\Event;
 use Symfony\Component\EventDispatcher\LegacyEventDispatcherProxy;
 use Symfony\Component\Notifier\Exception\IncompleteDsnException;
 use Symfony\Contracts\EventDispatcher\EventDispatcherInterface;
@@ -18,7 +19,6 @@
 
 /**
  * @author Konstantin Myakshin <molodchick@gmail.com>
- *
  * @author Fabien Potencier <fabien@symfony.com>
  *
  * @experimental in 5.0
@@ -30,7 +30,7 @@ abstract class AbstractTransportFactory implements TransportFactoryInterface
 
     public function __construct(EventDispatcherInterface $dispatcher = null, HttpClientInterface $client = null)
     {
-        $this->dispatcher = LegacyEventDispatcherProxy::decorate($dispatcher);
+        $this->dispatcher = class_exists(Event::class) ? LegacyEventDispatcherProxy::decorate($dispatcher) : $dispatcher;
         $this->client = $client;
     }
 
diff --git a/src/Symfony/Component/Security/Http/Firewall/ContextListener.php b/src/Symfony/Component/Security/Http/Firewall/ContextListener.php
index 69066552d7ca6..74dfd3a92ea14 100644
--- a/src/Symfony/Component/Security/Http/Firewall/ContextListener.php
+++ b/src/Symfony/Component/Security/Http/Firewall/ContextListener.php
@@ -12,6 +12,7 @@
 namespace Symfony\Component\Security\Http\Firewall;
 
 use Psr\Log\LoggerInterface;
+use Symfony\Component\EventDispatcher\Event;
 use Symfony\Component\EventDispatcher\LegacyEventDispatcherProxy;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Session\Session;
@@ -66,12 +67,7 @@ public function __construct(TokenStorageInterface $tokenStorage, iterable $userP
         $this->userProviders = $userProviders;
         $this->sessionKey = '_security_'.$contextKey;
         $this->logger = $logger;
-
-        if (null !== $dispatcher && class_exists(LegacyEventDispatcherProxy::class)) {
-            $this->dispatcher = LegacyEventDispatcherProxy::decorate($dispatcher);
-        } else {
-            $this->dispatcher = $dispatcher;
-        }
+        $this->dispatcher = class_exists(Event::class) ? LegacyEventDispatcherProxy::decorate($dispatcher) : $dispatcher;
 
         $this->trustResolver = $trustResolver ?: new AuthenticationTrustResolver(AnonymousToken::class, RememberMeToken::class);
         $this->sessionTrackerEnabler = $sessionTrackerEnabler;
diff --git a/src/Symfony/Component/Security/Http/composer.json b/src/Symfony/Component/Security/Http/composer.json
index 5cbf7c76618fb..52b59d083df5f 100644
--- a/src/Symfony/Component/Security/Http/composer.json
+++ b/src/Symfony/Component/Security/Http/composer.json
@@ -28,6 +28,7 @@
         "psr/log": "~1.0"
     },
     "conflict": {
+        "symfony/event-dispatcher": "<4.3",
         "symfony/security-csrf": "<4.4"
     },
     "suggest": {