diff --git a/edx_exams/apps/core/api.py b/edx_exams/apps/core/api.py
index d5c98791..11aac2ab 100644
--- a/edx_exams/apps/core/api.py
+++ b/edx_exams/apps/core/api.py
@@ -99,8 +99,11 @@ def update_attempt_status(attempt_id, to_status):
         if not allowed_to_start:
             raise ExamIllegalStatusTransition(error_msg)
 
-        attempt_obj.start_time = datetime.now(pytz.UTC)
-        attempt_obj.allowed_time_limit_mins = _calculate_allowed_mins(attempt_obj.user, attempt_obj.exam)
+        # Once start time, and end time by extension, has been set further transitions to started
+        # must not update this value
+        if not attempt_obj.start_time:
+            attempt_obj.start_time = datetime.now(pytz.UTC)
+            attempt_obj.allowed_time_limit_mins = _calculate_allowed_mins(attempt_obj.user, attempt_obj.exam)
 
     course_key = CourseKey.from_string(attempt_obj.exam.course_id)
     usage_key = UsageKey.from_string(attempt_obj.exam.content_id)
diff --git a/edx_exams/apps/core/tests/test_api.py b/edx_exams/apps/core/tests/test_api.py
index 718247ea..299697f7 100644
--- a/edx_exams/apps/core/tests/test_api.py
+++ b/edx_exams/apps/core/tests/test_api.py
@@ -407,6 +407,21 @@ def test_cannot_start_if_other_attempts_active(self):
             update_attempt_status(self.exam_attempt.id, ExamAttemptStatus.started)
         self.assertIn('another exam attempt is currently active!', str(exc.exception))
 
+    def test_ready_to_submit_to_started(self):
+        """
+        Test transition to started after initial start time has been set. This should not
+        alter the existing start time.
+        """
+        with freeze_time(timezone.now()):
+            self.exam_attempt.status = ExamAttemptStatus.ready_to_submit
+            self.exam_attempt.start_time = timezone.now()-timedelta(minutes=10)
+            self.exam_attempt.allowed_time_limit_mins = 30
+            self.exam_attempt.save()
+
+            update_attempt_status(self.exam_attempt.id, ExamAttemptStatus.started)
+            self.exam_attempt.refresh_from_db()
+            self.assertEqual(self.exam_attempt.start_time, timezone.now()-timedelta(minutes=10))
+
 
 class TestGetAttemptById(ExamsAPITestCase):
     """