diff --git a/system/Filters/Filters.php b/system/Filters/Filters.php
index 23028d455b94..9019c317aa1d 100644
--- a/system/Filters/Filters.php
+++ b/system/Filters/Filters.php
@@ -505,6 +505,12 @@ protected function processMethods()
         // @deprecated 4.5.0
         // @TODO remove this in the future.
         elseif (array_key_exists(strtolower($method), $this->config->methods)) {
+            @trigger_error(
+                'Setting lowercase HTTP method key "' . strtolower($method) . '" is deprecated.'
+                . ' Use uppercase HTTP method like "' . strtoupper($method) . '".',
+                E_USER_DEPRECATED
+            );
+
             $found  = true;
             $method = strtolower($method);
         }
diff --git a/system/Router/RouteCollection.php b/system/Router/RouteCollection.php
index 4195631f8ec3..05cb828976e7 100644
--- a/system/Router/RouteCollection.php
+++ b/system/Router/RouteCollection.php
@@ -1012,6 +1012,14 @@ public function match(array $verbs = [], string $from = '', $to = '', ?array $op
         }
 
         foreach ($verbs as $verb) {
+            if ($verb === strtolower($verb)) {
+                @trigger_error(
+                    'Passing lowercase HTTP method "' . $verb . '" is deprecated.'
+                    . ' Use uppercase HTTP method like "' . strtoupper($verb) . '".',
+                    E_USER_DEPRECATED
+                );
+            }
+
             /**
              * @TODO We should use correct uppercase verb.
              * @deprecated 4.5.0
diff --git a/system/Test/FeatureTestTrait.php b/system/Test/FeatureTestTrait.php
index 6e3d4a78fcf0..a254883865ad 100644
--- a/system/Test/FeatureTestTrait.php
+++ b/system/Test/FeatureTestTrait.php
@@ -51,6 +51,14 @@ protected function withRoutes(?array $routes = null)
             $collection->resetRoutes();
 
             foreach ($routes as $route) {
+                if ($route[0] === strtolower($route[0])) {
+                    @trigger_error(
+                        'Passing lowercase HTTP method "' . $route[0] . '" is deprecated.'
+                        . ' Use uppercase HTTP method like "' . strtoupper($route[0]) . '".',
+                        E_USER_DEPRECATED
+                    );
+                }
+
                 /**
                  * @TODO For backward compatibility. Remove strtolower() in the future.
                  * @deprecated 4.5.0