diff --git a/src/Symfony/Component/DependencyInjection/Compiler/CheckDefinitionValidityPass.php b/src/Symfony/Component/DependencyInjection/Compiler/CheckDefinitionValidityPass.php
index a43e0cd78f404..5bbadd6bdf12a 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/CheckDefinitionValidityPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/CheckDefinitionValidityPass.php
@@ -53,7 +53,7 @@ public function process(ContainerBuilder $container)
                          'The definition for "%s" has no class attribute, and appears to reference a '
                         .'class or interface in the global namespace. Leaving out the "class" attribute '
                         .'is only allowed for namespaced classes. Please specify the class attribute '
-                        .'explicitly to get rid of this error.',
+                        .'explicitly or change its service identifier to \\%1$s to get rid of this error.',
                         $id
                     ));
                 }
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveClassPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveClassPass.php
index c40d6f5d3d0f1..d380e62441e62 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveClassPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveClassPass.php
@@ -31,7 +31,7 @@ public function process(ContainerBuilder $container)
             if ($definition->isSynthetic() || null !== $definition->getClass()) {
                 continue;
             }
-            if (preg_match('/^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+(?:\\\\[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+)++$/', $id)) {
+            if (preg_match('/^(?:\\\\?[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+(?:\\\\[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+)++|\\\\[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+)$/', $id)) {
                 if ($definition instanceof ChildDefinition && !class_exists($id)) {
                     throw new InvalidArgumentException(sprintf('Service definition "%s" has a parent but no class, and its name looks like a FQCN. Either the class is missing or you want to inherit it from the parent service. To resolve this ambiguity, please rename this service to a non-FQCN (e.g. using dots), or create the missing class.', $id));
                 }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php b/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php
index efe9840e42c40..74245a0026b24 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php
@@ -1069,18 +1069,6 @@ public function testNoClassFromNonClassId()
         $container->compile();
     }
 
-    /**
-     * @expectedException \Symfony\Component\DependencyInjection\Exception\RuntimeException
-     * @expectedExceptionMessage The definition for "\foo" has no class.
-     */
-    public function testNoClassFromNsSeparatorId()
-    {
-        $container = new ContainerBuilder();
-
-        $definition = $container->register('\\foo');
-        $container->compile();
-    }
-
     public function testServiceLocator()
     {
         $container = new ContainerBuilder();
@@ -1129,6 +1117,26 @@ public function testPrivateServiceTriggersDeprecation()
 
         $container->get('bar');
     }
+
+    public function testNamespacedClassServiceWithRootNs()
+    {
+        $container = new ContainerBuilder();
+        $definition = $container->register($expected = '\\My\\DateTime');
+
+        $container->compile();
+
+        $this->assertSame($expected, $definition->getClass());
+    }
+
+    public function testClassServiceWithRootNs()
+    {
+        $container = new ContainerBuilder();
+        $definition = $container->register($expected = '\\DateTime');
+
+        $container->compile();
+
+        $this->assertSame($expected, $definition->getClass());
+    }
 }
 
 class FooClass