diff --git a/system/Router/RouteCollection.php b/system/Router/RouteCollection.php
index 8cdeadf0f904..c486f5baab20 100644
--- a/system/Router/RouteCollection.php
+++ b/system/Router/RouteCollection.php
@@ -1420,7 +1420,7 @@ protected function create(string $verb, string $from, $to, array $options = null
 		}
 
 		// If no namespace found, add the default namespace
-		if (is_string($to) && strpos($to, '\\') === false)
+		if (is_string($to) && (strpos($to, '\\') === false || strpos($to, '\\') > 0))
 		{
 			$namespace = $options['namespace'] ?? $this->defaultNamespace;
 			$to        = trim($namespace, '\\') . '\\' . $to;
diff --git a/tests/system/Router/RouteCollectionTest.php b/tests/system/Router/RouteCollectionTest.php
index 9439d46fbeb5..2a058d0d7a45 100644
--- a/tests/system/Router/RouteCollectionTest.php
+++ b/tests/system/Router/RouteCollectionTest.php
@@ -1324,7 +1324,7 @@ public function testRouteOverwritingDifferentSubdomains()
 		$routes->setDefaultMethod('index');
 		$routes->setHTTPVerb('get');
 
-		$routes->get('/', 'App\Controllers\Site\CDoc::index', ['subdomain' => 'doc', 'as' => 'doc_index']);
+		$routes->get('/', '\App\Controllers\Site\CDoc::index', ['subdomain' => 'doc', 'as' => 'doc_index']);
 		$routes->get('/', 'Home::index', ['subdomain' => 'dev']);
 
 		$expects = '\App\Controllers\Site\CDoc';
@@ -1344,7 +1344,7 @@ public function testRouteOverwritingTwoRules()
 		$routes->setDefaultController('Home');
 		$routes->setDefaultMethod('index');
 
-		$routes->get('/', 'App\Controllers\Site\CDoc::index', ['subdomain' => 'doc', 'as' => 'doc_index']);
+		$routes->get('/', '\App\Controllers\Site\CDoc::index', ['subdomain' => 'doc', 'as' => 'doc_index']);
 		$routes->get('/', 'Home::index');
 
 		// the second rule applies, so overwrites the first
@@ -1366,7 +1366,7 @@ public function testRouteOverwritingTwoRulesLastApplies()
 		$routes->setDefaultMethod('index');
 
 		$routes->get('/', 'Home::index');
-		$routes->get('/', 'App\Controllers\Site\CDoc::index', ['subdomain' => 'doc', 'as' => 'doc_index']);
+		$routes->get('/', '\App\Controllers\Site\CDoc::index', ['subdomain' => 'doc', 'as' => 'doc_index']);
 
 		$expects = '\App\Controllers\Site\CDoc';
 
@@ -1386,7 +1386,7 @@ public function testRouteOverwritingMatchingSubdomain()
 		$routes->setDefaultMethod('index');
 
 		$routes->get('/', 'Home::index', ['as' => 'ddd']);
-		$routes->get('/', 'App\Controllers\Site\CDoc::index', ['subdomain' => 'doc', 'as' => 'doc_index']);
+		$routes->get('/', '\App\Controllers\Site\CDoc::index', ['subdomain' => 'doc', 'as' => 'doc_index']);
 
 		$expects = '\App\Controllers\Site\CDoc';
 
@@ -1406,11 +1406,29 @@ public function testRouteOverwritingMatchingHost()
 		$routes->setDefaultMethod('index');
 
 		$routes->get('/', 'Home::index', ['as' => 'ddd']);
-		$routes->get('/', 'App\Controllers\Site\CDoc::index', ['hostname' => 'doc.domain.com', 'as' => 'doc_index']);
+		$routes->get('/', '\App\Controllers\Site\CDoc::index', ['hostname' => 'doc.domain.com', 'as' => 'doc_index']);
 
 		$expects = '\App\Controllers\Site\CDoc';
 
 		$this->assertEquals($expects, $router->handle('/'));
 	}
 
+	//--------------------------------------------------------------------
+	// Tests for router DefaultNameSpace issue
+	// @see https://github.com/codeigniter4/CodeIgniter4/issues/2423
+
+	public function testRouteDefaultNameSpace()
+	{
+		Services::request()->setMethod('get');
+		$routes = $this->getCollector();
+		$router = new Router($routes, Services::request());
+
+		$routes->setDefaultNamespace('App\Controllers');
+		$routes->get('/', 'Core\Home::index');
+
+		$expects = '\App\Controllers\Core\Home';
+
+		$this->assertEquals($expects, $router->handle('/'));
+	}
+
 }