diff --git a/app/main/forms.py b/app/main/forms.py
index f6f2716f3..66c506976 100644
--- a/app/main/forms.py
+++ b/app/main/forms.py
@@ -853,6 +853,11 @@ class LetterAddressForm(StripWhitespaceForm):
                 f'Last line of the address must be a real UK postcode'
             )
 
+        if address.has_invalid_characters:
+            raise ValidationError(
+                'Address lines must not start with any of the following characters: @ ( ) = [ ] ” \\ / ,'
+            )
+
 
 class EmailTemplateForm(BaseTemplateForm):
     subject = TextAreaField(
diff --git a/app/templates/views/check/row-errors.html b/app/templates/views/check/row-errors.html
index 839edddb7..4ffa028e4 100644
--- a/app/templates/views/check/row-errors.html
+++ b/app/templates/views/check/row-errors.html
@@ -90,6 +90,8 @@
                   {% else %}
                     Last line of the address must be a real UK postcode
                   {% endif %}
+                {% elif item.as_postal_address.has_invalid_characters %}
+                  Address lines must not start with any of the following characters: @ ( ) = [ ] ” \ / ,
                 {% endif %}
               </span>
             {% endcall %}
diff --git a/app/utils.py b/app/utils.py
index ebd1a89e9..a155395ab 100644
--- a/app/utils.py
+++ b/app/utils.py
@@ -733,7 +733,17 @@ LETTER_VALIDATION_MESSAGES = {
             f'Validation failed because the address must be no more '
             f'than {PostalAddress.MAX_LINES} lines long.'
         ),
-    }
+    },
+    'invalid-char-in-address': {
+        'title': 'There’s a problem with the address for this letter',
+        'detail': (
+            "Address lines must not start with any of the following characters: @ ( ) = [ ] ” \\ / ,"
+        ),
+        'summary': (
+            "Validation failed because address lines must not start with any of the "
+            "following characters: @ ( ) = [ ] ” \\ / ,"
+        ),
+    },
 }
 
 
diff --git a/tests/app/main/views/test_send.py b/tests/app/main/views/test_send.py
index 3d9975846..1733b05d5 100644
--- a/tests/app/main/views/test_send.py
+++ b/tests/app/main/views/test_send.py
@@ -614,6 +614,7 @@ def test_upload_csv_file_with_bad_postal_address_shows_check_page_with_errors(
             Firstname Lastname, 123 Example St., France
                               , 123 Example St., SW!A !AA
             "1\n2\n3\n4\n5\n6\n7\n8"
+            =Firstname Lastname, 123 Example St., SW1A 1AA
         '''
     )
 
@@ -630,7 +631,7 @@ def test_upload_csv_file_with_bad_postal_address_shows_check_page_with_errors(
         page.select_one('.banner-dangerous').text
     ) == (
         'There’s a problem with invalid.csv '
-        'You need to fix 4 addresses. '
+        'You need to fix 5 addresses. '
         'Skip to file contents'
     )
     assert [
@@ -647,6 +648,9 @@ def test_upload_csv_file_with_bad_postal_address_shows_check_page_with_errors(
 
         '6 Address must be no more than 7 lines long',
         '1 2 3 4 5 6 7 8',
+
+        '7 Address lines must not start with any of the following characters: @ ( ) = [ ] ” \\ / ,',
+        '=Firstname Lastname 123 Example St. SW1A 1AA',
     ]
 
 
@@ -2538,6 +2542,11 @@ def test_send_one_off_letter_address_populates_address_fields_in_session(
         ['international_letters'],
         'Last line of the address must be a UK postcode or another country',
     ),
+    (
+        'a\n(b\nSW1A 1AA',
+        [],
+        'Address lines must not start with any of the following characters: @ ( ) = [ ] ” \\ / ,',
+    ),
 ])
 def test_send_one_off_letter_address_rejects_bad_addresses(
     client_request,
diff --git a/tests/app/test_utils.py b/tests/app/test_utils.py
index bedb4eb97..a53723f00 100644
--- a/tests/app/test_utils.py
+++ b/tests/app/test_utils.py
@@ -545,6 +545,18 @@ def test_get_letter_validation_error_for_unknown_error():
             'Validation failed because the address must be no more than 7 lines long.'
         ),
     ),
+    (
+        'invalid-char-in-address',
+        None,
+        'There’s a problem with the address for this letter',
+        (
+            'Address lines must not start with any of the following characters: @ ( ) = [ ] ” \\ / ,'
+        ),
+        (
+            'Validation failed because address lines must not start with any of the following '
+            'characters: @ ( ) = [ ] ” \\ / ,'
+        ),
+    ),
 ])
 def test_get_letter_validation_error_for_known_errors(
     client_request,