diff --git a/app/broadcast_message/rest.py b/app/broadcast_message/rest.py
index e75a19079..38783e099 100644
--- a/app/broadcast_message/rest.py
+++ b/app/broadcast_message/rest.py
@@ -1,7 +1,7 @@
 from datetime import datetime
 
 import iso8601
-from flask import Blueprint, jsonify, request, current_app
+from flask import Blueprint, jsonify, request, current_app, abort
 
 from app.config import QueueNames
 from app.dao.templates_dao import dao_get_template_by_id_and_service_id
@@ -85,6 +85,12 @@ def update_broadcast_message(service_id, broadcast_message_id):
 
     broadcast_message = dao_get_broadcast_message_by_id_and_service_id(broadcast_message_id, service_id)
 
+    if broadcast_message.status not in BroadcastStatusType.PRE_BROADCAST_STATUSES:
+        abort(
+            400,
+            f'Cannot update broadcast_message {broadcast_message.id} while it has status {broadcast_message.status}'
+        )
+
     if 'personalisation' in data:
         broadcast_message.personalisation = data['personalisation']
     if 'starts_at' in data:
diff --git a/app/models.py b/app/models.py
index d447505b2..6f5b9a2b7 100644
--- a/app/models.py
+++ b/app/models.py
@@ -2172,6 +2172,10 @@ class BroadcastStatusType(db.Model):
 
     STATUSES = [DRAFT, PENDING_APPROVAL, REJECTED, BROADCASTING, COMPLETED, CANCELLED, TECHNICAL_FAILURE]
 
+    # a broadcast message can be edited while in one of these states
+    PRE_BROADCAST_STATUSES = [DRAFT, PENDING_APPROVAL, REJECTED]
+    LIVE_STATUSES = [BROADCASTING, COMPLETED, CANCELLED]
+
     name = db.Column(db.String, primary_key=True)
 
 
diff --git a/tests/app/broadcast_message/test_rest.py b/tests/app/broadcast_message/test_rest.py
index 39627e9b1..baa14834a 100644
--- a/tests/app/broadcast_message/test_rest.py
+++ b/tests/app/broadcast_message/test_rest.py
@@ -132,9 +132,14 @@ def test_create_broadcast_message_400s_if_json_schema_fails_validation(
     assert response['errors'] == expected_errors
 
 
-def test_update_broadcast_message(admin_request, sample_service):
+@pytest.mark.parametrize('status', [
+    BroadcastStatusType.DRAFT,
+    BroadcastStatusType.PENDING_APPROVAL,
+    BroadcastStatusType.REJECTED,
+])
+def test_update_broadcast_message_allows_edit_while_not_yet_live(admin_request, sample_service, status):
     t = create_template(sample_service, BROADCAST_TYPE)
-    bm = create_broadcast_message(t, areas=['manchester'])
+    bm = create_broadcast_message(t, areas=['manchester'], status=status)
 
     response = admin_request.post(
         'broadcast_message.update_broadcast_message',
@@ -149,6 +154,26 @@ def test_update_broadcast_message(admin_request, sample_service):
     assert response['updated_at'] is not None
 
 
+@pytest.mark.parametrize('status', [
+    BroadcastStatusType.BROADCASTING,
+    BroadcastStatusType.CANCELLED,
+    BroadcastStatusType.COMPLETED,
+    BroadcastStatusType.TECHNICAL_FAILURE,
+])
+def test_update_broadcast_message_doesnt_allow_edits_after_broadcast_goes_live(admin_request, sample_service, status):
+    t = create_template(sample_service, BROADCAST_TYPE)
+    bm = create_broadcast_message(t, areas=['manchester'], status=status)
+
+    response = admin_request.post(
+        'broadcast_message.update_broadcast_message',
+        _data={'areas': ['london', 'glasgow']},
+        service_id=t.service_id,
+        broadcast_message_id=bm.id,
+        _expected_status=400
+    )
+    assert f'status {status}' in response['message']
+
+
 def test_update_broadcast_message_sets_finishes_at_separately(admin_request, sample_service):
     t = create_template(sample_service, BROADCAST_TYPE)
     bm = create_broadcast_message(t, areas=['manchester'])
diff --git a/tests/app/db.py b/tests/app/db.py
index 190ef0c0c..0fc430102 100644
--- a/tests/app/db.py
+++ b/tests/app/db.py
@@ -1011,7 +1011,7 @@ def create_broadcast_message(
         template_id=template.id,
         template_version=template.version,
         personalisation=personalisation,
-        status=BroadcastStatusType.DRAFT,
+        status=status,
         starts_at=starts_at,
         finishes_at=finishes_at,
         created_by_id=created_by.id if created_by else template.created_by_id,