diff --git a/uc_cart/uc_cart.pages.inc b/uc_cart/uc_cart.pages.inc
index d3a2f09..e32c10d 100644
--- a/uc_cart/uc_cart.pages.inc
+++ b/uc_cart/uc_cart.pages.inc
@@ -164,6 +164,19 @@ function uc_cart_checkout() {
     }
   }
 
+  // Check if the cart is valid for checkout.
+  $cart_validity = new stdClass();
+  $cart_validity->validity = TRUE;
+  $cart_validity->messages = array();
+  rules_invoke_event('uc_cart_checkout_start', $order, $cart_validity);
+  if (!$cart_validity->validity) {
+    drupal_set_message(t('You can not check out.'), 'error');
+    foreach ($cart_validity->messages as $message) {
+      drupal_set_message(check_plain($message), 'error');
+    }
+    drupal_goto('cart');
+  }
+
   return drupal_get_form('uc_cart_checkout_form', $order);
 }
 
diff --git a/uc_cart/uc_cart.rules.inc b/uc_cart/uc_cart.rules.inc
index 26b4003..a3e599d 100644
--- a/uc_cart/uc_cart.rules.inc
+++ b/uc_cart/uc_cart.rules.inc
@@ -7,6 +7,29 @@
  */
 
 /**
+ * Implements hook_rules_data_info().
+ */
+function uc_cart_rules_data_info() {
+  $types['uc_cart_validity'] = array(
+    'label' => t('Ubercart checkout validity'), 
+    'wrap' => TRUE, 
+    'group' => t('Ubercart'),
+    'property info' => array(
+      'validity' => array(
+        'type' => 'boolean',
+        'label' => t('Whether the checkout can proceed')
+      ),
+      'messages' => array(
+        'type' => 'list<text>',
+        'label' => 'List of error messages',
+      ),
+    ),
+  );
+
+  return $types;
+}
+
+/**
  * Implements hook_rules_event_info().
  */
 function uc_cart_rules_event_info() {
@@ -21,5 +44,64 @@ function uc_cart_rules_event_info() {
     ),
   );
 
+  $events['uc_cart_checkout_start'] = array(
+    'label' => t('Customer starts checkout'),
+    'group' => t('Cart'),
+    'variables' => array(
+      'order' => array(
+        'type' => 'uc_order',
+        'label' => t('Order'),
+      ),
+      'validity' => array(
+        'type' => 'uc_cart_validity',
+        'label' => t('Validity'),
+      ),
+    ),
+  );
+
   return $events;
 }
+
+/**
+ * Implements hook_rules_action_info().
+ */
+function uc_cart_rules_action_info() {
+
+  $actions['uc_cart_checkout_abort'] = array(
+    'label' => t('Indicate that Ubercart checkout should not start'),
+    'group' => t('Cart'),
+    'base' => 'uc_cart_checkout_abort',
+    'parameter' => array(
+      'cart_validity' => array(
+        'type' => 'uc_cart_validity',
+        'label' => t('Cart validity'),
+      ),
+      'message' => array(
+        'type' => 'text',
+        'label' => 'error message',
+        'translatable' => TRUE,
+      ),
+    ),
+  ); 
+
+  return $actions;
+}
+
+/**
+ * Indicate that a customer may not proceed to checkout.
+ *
+ * @param object $cart_validity
+ *   Object to represent the validity of the cart.
+ *   Updating this object sends information back to the function
+ *   that invoked the event.
+ *   - boolean $validity: Whether checkout can start.
+ *   - array $messages: List of error messages.
+ * @param string $message
+ *   New error message to add to the list of messages.
+ */
+function uc_cart_checkout_abort($cart_validity, $message) {
+  $cart_validity->validity = FALSE;
+  if (!empty($message)) {
+    $cart_validity->messages[] = $message;
+  }
+}