diff --git a/changelog.d/1900.changed.md b/changelog.d/1900.changed.md
new file mode 100644
index 000000000..a899cdadd
--- /dev/null
+++ b/changelog.d/1900.changed.md
@@ -0,0 +1 @@
+Blocked editing of start time for planned maintenance tasks that have already started
diff --git a/src/argus/plannedmaintenance/models.py b/src/argus/plannedmaintenance/models.py
index abab95d32..ae9c558ba 100644
--- a/src/argus/plannedmaintenance/models.py
+++ b/src/argus/plannedmaintenance/models.py
@@ -137,6 +137,8 @@ def clean(self):
                 raise ValidationError(
                     f"This planned maintenance task is no longer modifiable as it ended more than {hours} hours ago."
                 )
+            if not old.future and old.start_time != self.start_time:
+                raise ValidationError("The start time cannot be modified after the task has already started.")
 
     def save(self, *args, **kwargs):
         self.full_clean()
diff --git a/tests/plannedmaintenance/test_models.py b/tests/plannedmaintenance/test_models.py
index 00561663b..643477ee3 100644
--- a/tests/plannedmaintenance/test_models.py
+++ b/tests/plannedmaintenance/test_models.py
@@ -160,3 +160,31 @@ def test_given_current_pm_cancel_sets_end_time(self):
 
         self.assertNotEqual(self.current_pm.end_time, current_pm_end_time)
         self.assertLess(self.current_pm.end_time, timezone.now())
+
+    def test_given_task_has_not_started_then_can_modify_start_time(self):
+        new_start = self.future_pm.start_time + timedelta(hours=1)
+        self.future_pm.start_time = new_start
+        self.future_pm.save()
+        self.future_pm.refresh_from_db()
+        self.assertEqual(self.future_pm.start_time, new_start)
+
+    def test_given_task_has_already_started_then_cannot_modify_start_time(self):
+        original_start = self.current_pm.start_time
+        self.current_pm.start_time = timezone.now() + timedelta(hours=1)
+        with self.assertRaises(ValidationError):
+            self.current_pm.save()
+        self.current_pm.refresh_from_db()
+        self.assertEqual(self.current_pm.start_time, original_start)
+
+    def test_given_task_has_recently_ended_then_cannot_modify_start_time(self):
+        now = timezone.now()
+        recently_ended_pm = PlannedMaintenanceFactory(
+            start_time=now - timedelta(hours=2),
+            end_time=now - timedelta(minutes=5),
+        )
+        original_start = recently_ended_pm.start_time
+        recently_ended_pm.start_time = now + timedelta(hours=1)
+        with self.assertRaises(ValidationError):
+            recently_ended_pm.save()
+        recently_ended_pm.refresh_from_db()
+        self.assertEqual(recently_ended_pm.start_time, original_start)