diff --git a/app/broadcast_message/broadcast_message_schema.py b/app/broadcast_message/broadcast_message_schema.py
index 09eff203b..fc4527511 100644
--- a/app/broadcast_message/broadcast_message_schema.py
+++ b/app/broadcast_message/broadcast_message_schema.py
@@ -15,7 +15,7 @@ create_broadcast_message_schema = {
         'finishes_at': {'type': 'string', 'format': 'datetime'},
         'areas': {"type": "array", "items": {"type": "string"}},
         'simple_polygons': {"type": "array", "items": {"type": "array"}},
-        'content': {'type': 'string', 'minLength': 1, 'maxLength': 1395},
+        'content': {'type': 'string', 'minLength': 1},
         'reference': {'type': 'string', 'minLength': 1, 'maxLength': 255},
     },
     'required': ['service_id', 'created_by'],
diff --git a/app/broadcast_message/rest.py b/app/broadcast_message/rest.py
index 31e3a5a4d..26221b89a 100644
--- a/app/broadcast_message/rest.py
+++ b/app/broadcast_message/rest.py
@@ -2,6 +2,7 @@ from datetime import datetime
 
 import iso8601
 from flask import Blueprint, current_app, jsonify, request
+from notifications_utils.template import BroadcastMessageTemplate
 
 from app.broadcast_message.broadcast_message_schema import (
     create_broadcast_message_schema,
@@ -116,6 +117,19 @@ def create_broadcast_message(service_id):
         reference = None
     else:
         template, content, reference = None, data['content'], data['reference']
+        temporary_template = BroadcastMessageTemplate.from_content(content)
+        if temporary_template.content_too_long:
+            raise InvalidRequest(
+                (
+                    f'Content must be '
+                    f'{temporary_template.max_content_count:,.0f} '
+                    f'characters or fewer'
+                ) + (
+                    ' (because it could not be GSM7 encoded)'
+                    if temporary_template.non_gsm_characters else ''
+                ),
+                status_code=400,
+            )
 
     broadcast_message = BroadcastMessage(
         service_id=service.id,
diff --git a/tests/app/broadcast_message/test_rest.py b/tests/app/broadcast_message/test_rest.py
index 49550c515..438aedaf4 100644
--- a/tests/app/broadcast_message/test_rest.py
+++ b/tests/app/broadcast_message/test_rest.py
@@ -192,6 +192,40 @@ def test_create_broadcast_message_400s_if_json_schema_fails_validation(
     assert response['errors'] == expected_errors
 
 
+@pytest.mark.parametrize('content, expected_status, expected_errors', (
+    ('a', 201, None),
+    ('a' * 1_395, 201, None),
+    ('a\r\n' * 697, 201, None),  # 1,394 chars – new lines normalised to \n
+    ('a' * 1_396, 400, (
+        'Content must be 1,395 characters or fewer'
+    )),
+    ('ŵ' * 615, 201, None),
+    ('ŵ' * 616, 400, (
+        'Content must be 615 characters or fewer '
+        '(because it could not be GSM7 encoded)'
+    )),
+))
+def test_create_broadcast_message_400s_if_content_too_long(
+    admin_request,
+    sample_broadcast_service,
+    content,
+    expected_status,
+    expected_errors,
+):
+    response = admin_request.post(
+        'broadcast_message.create_broadcast_message',
+        service_id=sample_broadcast_service.id,
+        _data={
+            'service_id': str(sample_broadcast_service.id),
+            'created_by': str(sample_broadcast_service.created_by_id),
+            'reference': 'abc123',
+            'content': content,
+        },
+        _expected_status=expected_status,
+    )
+    assert response.get('message') == expected_errors
+
+
 @freeze_time('2020-01-01')
 def test_create_broadcast_message_can_be_created_from_content(admin_request, sample_broadcast_service):
     response = admin_request.post(