diff --git a/services/nginx/app/objects/order_bookings_o.php b/services/nginx/app/objects/order_bookings_o.php
index 12159b2b..8888020f 100644
--- a/services/nginx/app/objects/order_bookings_o.php
+++ b/services/nginx/app/objects/order_bookings_o.php
@@ -369,6 +369,7 @@ class order_bookings_o extends db
             return;
         }
 
+        $this->requireLinkedOrderMatchesBooking($order);
         $normalizedSafetySeal = orders_o::normalizeSafetySealValue($safety_seal);
         if ($normalizedSafetySeal !== null) {
             $order->setSafetySealValue($normalizedSafetySeal);
@@ -467,14 +468,34 @@ class order_bookings_o extends db
         return $order;
     }
 
+    /**
+     * @throws Exception
+     */
+    private function requireLinkedOrderMatchesBooking(orders_o $order): void
+    {
+        self::requireSelected();
+
+        $bookingCustomerNumber = (int)$this->customer_number->value();
+        $bookingDepartmentId = (int)$this->department->value();
+        $orderCustomerId = (int)$order->customer_id->value();
+        $orderDepartmentId = (int)$order->department_id->value();
+
+        if ($orderCustomerId !== $bookingCustomerNumber || $orderDepartmentId !== $bookingDepartmentId) {
+            throw new Exception('Linked order does not match booking customer or department');
+        }
+    }
+
     /**
      * @throws Exception
      */
     protected function attachWashCertificate(int $user_id, ?string $safety_seal = null): void
     {
         self::requireSelected();
+        $order = $this->getOrder();
+        $this->requireLinkedOrderMatchesBooking($order);
+
         // Check if the order already has a wash certificate attached
-        if ($this->getOrder()->hasWashCertificateAttached()) {
+        if ($order->hasWashCertificateAttached()) {
             return;
         }
         // Get the operator name
diff --git a/services/nginx/app/tests/Unit/Orders/OrderBookingsCompletionDedupTest.php b/services/nginx/app/tests/Unit/Orders/OrderBookingsCompletionDedupTest.php
index 737dbca4..d62df2b2 100644
--- a/services/nginx/app/tests/Unit/Orders/OrderBookingsCompletionDedupTest.php
+++ b/services/nginx/app/tests/Unit/Orders/OrderBookingsCompletionDedupTest.php
@@ -15,8 +15,12 @@ if (!class_exists('OrderBookingsCompletionOrderDouble')) {
         {
             $this->id = -1;
             $this->booking_id = new object_property('orders', -1, 'booking_id', 'int', false);
+            $this->customer_id = new object_property('orders', -1, 'customer_id', 'int', true);
+            $this->department_id = new object_property('orders', -1, 'department_id', 'int', true);
             $this->safety_seal = new object_property('orders', -1, 'safety_seal', 'string', false);
             $this->completed_at = new object_property('orders', -1, 'completed_at', 'timestamp', false);
+            $this->customer_id->set(111111);
+            $this->department_id->set(10);
         }
 
         public bool $washCertificateAttached = false;
@@ -52,8 +56,12 @@ if (!class_exists('OrderBookingsCompletionDouble')) {
         {
             $this->id = -1;
             $this->linkedOrder = $linkedOrder;
+            $this->customer_number = new object_property('order_bookings', -1, 'customer_number', 'int', true);
+            $this->department = new object_property('order_bookings', -1, 'department', 'int', true);
             $this->order_id = new object_property('order_bookings', -1, 'order_id', 'int', false);
             $this->items = new object_property('order_bookings', -1, 'items', 'json', false);
+            $this->customer_number->set(111111);
+            $this->department->set(10);
             $this->items->set([]);
         }
 
@@ -91,7 +99,7 @@ if (!class_exists('OrderBookingsCompletionDouble')) {
     }
 }
 
-it('attaches and emails a wash certificate when a booking is already linked to a pos order without one', function (): void {
+it('attaches and emails a wash certificate when a booking is already linked to a matching pos order without one', function (): void {
     $order = new OrderBookingsCompletionOrderDouble();
     $booking = new OrderBookingsCompletionDouble($order);
     $booking->order_id->set(321);
@@ -104,6 +112,22 @@ it('attaches and emails a wash certificate when a booking is already linked to a
     expect($booking->sendCalls)->toBe(1);
 });
 
+it('rejects wash certificate completion when the linked pos order belongs to another booking context', function (): void {
+    $order = new OrderBookingsCompletionOrderDouble();
+    $order->customer_id->set(222222);
+    $order->department_id->set(99);
+
+    $booking = new OrderBookingsCompletionDouble($order);
+    $booking->order_id->set(321);
+    $booking->containsWashCertificate = true;
+
+    expect(fn() => $booking->completeBooking(77, 'LINKED-SEAL'))
+        ->toThrow(Exception::class, 'Linked order does not match booking customer or department');
+    expect($order->getSafetySealValue())->toBeNull();
+    expect($booking->attachCalls)->toBe(0);
+    expect($booking->sendCalls)->toBe(0);
+});
+
 it('uses the linked pos order wash certificate item added during mobile completion', function (): void {
     $order = new OrderBookingsCompletionOrderDouble();
     $order->containsWashCertificate = true;