diff --git a/app/main/forms.py b/app/main/forms.py
index de0bcd8e6..77b228fe7 100644
--- a/app/main/forms.py
+++ b/app/main/forms.py
@@ -32,6 +32,7 @@ from wtforms.validators import DataRequired, Length, Optional, Regexp
 from app.main.validators import (
     Blacklist,
     CsvFileValidator,
+    DoesNotStartWithDoubleZero,
     LettersNumbersAndFullStopsOnly,
     NoCommasInPlaceHolders,
     OnlyGSMCharacters,
@@ -594,6 +595,7 @@ class ServiceSmsSenderForm(StripWhitespaceForm):
             Length(max=11, message="Enter 11 characters or fewer"),
             Length(min=3, message="Enter 3 characters or more"),
             LettersNumbersAndFullStopsOnly(),
+            DoesNotStartWithDoubleZero(),
         ]
     )
     is_default = BooleanField("Make this text message sender the default")
diff --git a/app/main/validators.py b/app/main/validators.py
index 3ea714b1f..b82964e9e 100644
--- a/app/main/validators.py
+++ b/app/main/validators.py
@@ -92,3 +92,13 @@ class LettersNumbersAndFullStopsOnly:
     def __call__(self, form, field):
         if field.data and not re.match(self.regex, field.data):
             raise ValidationError(self.message)
+
+
+class DoesNotStartWithDoubleZero:
+
+    def __init__(self, message="Can't start with 00"):
+        self.message = message
+
+    def __call__(self, form, field):
+        if field.data and field.data.startswith("00"):
+            raise ValidationError(self.message)
diff --git a/tests/app/main/test_validators.py b/tests/app/main/test_validators.py
index bbfc7e59f..515a545c0 100644
--- a/tests/app/main/test_validators.py
+++ b/tests/app/main/test_validators.py
@@ -217,6 +217,10 @@ def test_sms_sender_form_validation(
     form.validate()
     assert 'Enter 3 characters or more' == form.errors['sms_sender'][0]
 
-    form.sms_sender.data = '000'
+    form.sms_sender.data = '111'
     form.validate()
     assert not form.errors
+
+    form.sms_sender.data = '00111222333'
+    form.validate()
+    assert "Can't start with 00" == form.errors['sms_sender'][0]
diff --git a/tests/app/main/views/test_service_settings.py b/tests/app/main/views/test_service_settings.py
index 2ec13b3a9..8b5b11211 100644
--- a/tests/app/main/views/test_service_settings.py
+++ b/tests/app/main/views/test_service_settings.py
@@ -873,6 +873,7 @@ def test_incorrect_letter_contact_block_input(
     ('abcdefghijkhgkg', 'Enter 11 characters or fewer'),
     (' ¯\_(ツ)_/¯ ', 'Use letters and numbers only'),
     ('blood.co.uk', None),
+    ('00123', "Can't start with 00")
 ])
 def test_incorrect_sms_sender_input(
     sms_sender_input,