diff --git a/addons/sale_timesheet/models/sale_order.py b/addons/sale_timesheet/models/sale_order.py
index 4d9d707f333..2c80fa04888 100644
--- a/addons/sale_timesheet/models/sale_order.py
+++ b/addons/sale_timesheet/models/sale_order.py
@@ -177,6 +177,19 @@ class SaleOrderLine(models.Model):
                 line.order_id.message_post(body=msg_body)
         return line
 
+    @api.multi
+    def write(self, values):
+        result = super(SaleOrderLine, self).write(values)
+        # changing the ordered quantity should change the planned hours on the
+        # task, whatever the SO state. It will be blocked by the super in case
+        # of a locked sale order.
+        if 'product_uom_qty' in values:
+            for line in self:
+                if line.task_id:
+                    planned_hours = line._convert_qty_company_hours()
+                    line.task_id.write({'planned_hours': planned_hours})
+        return result
+
     ###########################################
     # Service : Project and task generation
     ###########################################
diff --git a/addons/sale_timesheet/tests/test_sale_service.py b/addons/sale_timesheet/tests/test_sale_service.py
index 0fd318085a0..e0b9c44669e 100644
--- a/addons/sale_timesheet/tests/test_sale_service.py
+++ b/addons/sale_timesheet/tests/test_sale_service.py
@@ -373,3 +373,28 @@ class TestSaleService(TestCommonSaleTimesheetNoChart):
         subtask.write({'project_id': self.project_global.id})
 
         self.assertEqual(subtask.sale_line_id, task.sale_line_id, "A child task should always have the same SO line than its mother, even when changing project")
+
+    def test_change_ordered_qty(self):
+        """ Changing the ordered quantity of a SO line that have created a task should update the planned hours of this task """
+        sale_order_line = self.env['sale.order.line'].create({
+            'order_id': self.sale_order.id,
+            'name': self.product_delivery_timesheet2.name,
+            'product_id': self.product_delivery_timesheet2.id,
+            'product_uom_qty': 50,
+            'product_uom': self.product_delivery_timesheet2.uom_id.id,
+            'price_unit': self.product_delivery_timesheet2.list_price
+        })
+
+        self.sale_order.action_confirm()
+        self.assertEqual(sale_order_line.product_uom_qty, sale_order_line.task_id.planned_hours, "The planned hours should be the same as the ordered quantity of the native SO line")
+
+        sale_order_line.write({'product_uom_qty': 20})
+        self.assertEqual(sale_order_line.product_uom_qty, sale_order_line.task_id.planned_hours, "The planned hours should have changed when updating the ordered quantity of the native SO line")
+
+        self.sale_order.action_cancel()
+        sale_order_line.write({'product_uom_qty': 30})
+        self.assertEqual(sale_order_line.product_uom_qty, sale_order_line.task_id.planned_hours, "The planned hours should have changed when updating the ordered quantity, even after SO cancellation")
+
+        self.sale_order.action_done()
+        with self.assertRaises(UserError):
+            sale_order_line.write({'product_uom_qty': 20})