diff --git a/app/main/forms.py b/app/main/forms.py
index eca50ab26..46592daf9 100644
--- a/app/main/forms.py
+++ b/app/main/forms.py
@@ -769,6 +769,11 @@ class LetterAddressForm(StripWhitespaceForm):
                 f'Address must be no more than {PostalAddress.MAX_LINES} lines long'
             )
 
+        if not address.postcode:
+            raise ValidationError(
+                f'Last line of the address must be a real UK postcode'
+            )
+
 
 class EmailTemplateForm(BaseTemplateForm):
     subject = TextAreaField(
diff --git a/tests/app/main/views/test_send.py b/tests/app/main/views/test_send.py
index 8547bec48..a43a06420 100644
--- a/tests/app/main/views/test_send.py
+++ b/tests/app/main/views/test_send.py
@@ -2204,38 +2204,38 @@ def test_send_one_off_letter_address_shows_form(
 
 @pytest.mark.parametrize(['form_data', 'expected_placeholders'], [
     # minimal
-    ('\n'.join(['a', 'b', 'c']), {
+    ('\n'.join(['a', 'b', 'sw1a1aa']), {
         'address_line_1': 'a',
         'address_line_2': 'b',
         'address_line_3': '',
         'address_line_4': '',
         'address_line_5': '',
         'address_line_6': '',
-        'address_line_7': 'c',
-        'postcode': 'c',
+        'address_line_7': 'SW1A 1AA',
+        'postcode': 'SW1A 1AA',
     }),
     # maximal
-    ('\n'.join(['a', 'b', 'c', 'd', 'e', 'f', 'g']), {
+    ('\n'.join(['a', 'b', 'c', 'd', 'e', 'f', 'sw1a1aa']), {
         'address_line_1': 'a',
         'address_line_2': 'b',
         'address_line_3': 'c',
         'address_line_4': 'd',
         'address_line_5': 'e',
         'address_line_6': 'f',
-        'address_line_7': 'g',
-        'postcode': 'g',
+        'address_line_7': 'SW1A 1AA',
+        'postcode': 'SW1A 1AA',
     }),
     # it ignores empty lines and strips whitespace from each line.
     # It also strips extra whitespace from the middle of lines.
-    ('\n  a\ta  \n\n\n      \n\n\n\nb  b   \r\nc', {
+    ('\n  a\ta  \n\n\n      \n\n\n\nb  b   \r\n sw1a1aa \n\n', {
         'address_line_1': 'a\ta',
         'address_line_2': 'b b',
         'address_line_3': '',
         'address_line_4': '',
         'address_line_5': '',
         'address_line_6': '',
-        'address_line_7': 'c',
-        'postcode': 'c',
+        'address_line_7': 'SW1A 1AA',
+        'postcode': 'SW1A 1AA',
     }),
 ])
 def test_send_one_off_letter_address_populates_address_fields_in_session(
@@ -2271,6 +2271,7 @@ def test_send_one_off_letter_address_populates_address_fields_in_session(
     ('', 'Cannot be empty'),
     ('a\n\n\n\nb', 'Address must be at least 3 lines long'),
     ('\n'.join(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']), 'Address must be no more than 7 lines long'),
+    ('\n'.join(['a', 'b', 'c', 'd', 'e', 'f', 'g']), 'Last line of the address must be a real UK postcode'),
 ])
 def test_send_one_off_letter_address_rejects_bad_addresses(
     client_request,
@@ -2309,7 +2310,7 @@ def test_send_one_off_letter_address_goes_to_next_placeholder(client_request, mo
         'main.send_one_off_letter_address',
         service_id=SERVICE_ONE_ID,
         template_id=template_data['id'],
-        _data={'address': 'a\nb\nc'},
+        _data={'address': 'a\nb\nSW1A 1AA'},
         # step 0-6 represent address line 1-6 and postcode. step 7 is the first non address placeholder
         _expected_redirect=url_for(
             'main.send_one_off_step',