diff --git a/src/Factory/AdminContextFactory.php b/src/Factory/AdminContextFactory.php
index 493cd7f7c4..72881a25ff 100644
--- a/src/Factory/AdminContextFactory.php
+++ b/src/Factory/AdminContextFactory.php
@@ -77,9 +77,10 @@ private function getDashboardDto(Request $request, DashboardControllerInterface
 
         foreach ($dashboardControllerRoutes as $routeName => $controller) {
             if ($controller === $dashboardController) {
-                // if present, remove the suffix of i18n route names (it's a two-letter locale at the end
-                // of the route name; e.g. 'dashboard.en' -> remove '.en', 'admin.index.es' -> remove '.es')
-                $dashboardRouteName = preg_replace('~\.\w{2}$~', '', $routeName);
+                // if present, remove the suffix of i18n route names (it's an ISO639 or ISO3166 locale at the end
+                // of the route name for ; e.g. 'dashboard.en' -> remove '.en', 'admin.index.es' -> remove '.es'
+                // or 'dashboard.en_GB' -> remove '.en_GB', 'admin.index.es_ES' -> remove '.es_ES')
+                $dashboardRouteName = preg_replace('~\.\w{2}(_\w{2})?$~', '', $routeName);
 
                 break;
             }
diff --git a/src/Router/AdminUrlGenerator.php b/src/Router/AdminUrlGenerator.php
index ae6afbbd13..538722c598 100644
--- a/src/Router/AdminUrlGenerator.php
+++ b/src/Router/AdminUrlGenerator.php
@@ -294,9 +294,10 @@ public function generateUrl(): string
             $this->dashboardRoute = $this->dashboardControllerRegistry->getFirstDashboardRoute();
         }
 
-        // if present, remove the suffix of i18n route names (it's a two-letter locale at the end
-        // of the route name; e.g. 'dashboard.en' -> remove '.en', 'admin.index.es' -> remove '.es')
-        $this->dashboardRoute = preg_replace('~\.\w{2}$~', '', $this->dashboardRoute);
+        // if present, remove the suffix of i18n route names (it's an ISO639 or ISO3166 locale at the end
+        // of the route name for ; e.g. 'dashboard.en' -> remove '.en', 'admin.index.es' -> remove '.es'
+        // or 'dashboard.en_GB' -> remove '.en_GB', 'admin.index.es_ES' -> remove '.es_ES')
+        $this->dashboardRoute = preg_replace('~\.\w{2}(_\w{2})?$~', '', $this->dashboardRoute);
 
         // this removes any parameter with a NULL value
         $routeParameters = array_filter(