diff --git a/app/main/forms.py b/app/main/forms.py
index 403e3317c..243534d9b 100644
--- a/app/main/forms.py
+++ b/app/main/forms.py
@@ -1742,6 +1742,8 @@ class EstimateUsageForm(StripWhitespaceForm):
 
 class AdminProviderRatioForm(Form):
     def __init__(self, providers):
+        self._providers = providers
+
         # hack: https://github.com/wtforms/wtforms/issues/736
         self._unbound_fields = [
             (
@@ -1760,6 +1762,25 @@ class AdminProviderRatioForm(Form):
             for provider in providers
         })
 
+    def validate(self):
+        if not super().validate():
+            return False
+
+        total = sum(
+            getattr(self, provider['identifier']).data
+            for provider in self._providers
+        )
+
+        if total == 100:
+            return True
+
+        for provider in self._providers:
+            getattr(self, provider['identifier']).errors += [
+                'Must add up to 100%'
+            ]
+
+        return False
+
 
 class ServiceContactDetailsForm(StripWhitespaceForm):
     contact_details_type = RadioField(
diff --git a/tests/app/main/views/test_providers.py b/tests/app/main/views/test_providers.py
index 1770493ff..aefacdffc 100644
--- a/tests/app/main/views/test_providers.py
+++ b/tests/app/main/views/test_providers.py
@@ -337,3 +337,46 @@ def test_edit_sms_provider_ratio_submit(
     )
 
     assert mock_update_provider.call_args_list == expected_calls
+
+
+@pytest.mark.parametrize('post_data, expected_error', [
+    (
+        {
+            sms_provider_1['identifier']: 90,
+            sms_provider_2['identifier']: 20
+        },
+        "Must add up to 100%"
+    ),
+    (
+        {
+            sms_provider_1['identifier']: 101,
+            sms_provider_2['identifier']: 20
+        },
+        "Must be between 0 and 100"
+    ),
+])
+def test_edit_sms_provider_submit_invalid_percentages(
+    client_request,
+    platform_admin_user,
+    mocker,
+    post_data,
+    expected_error,
+    stub_providers,
+):
+    mocker.patch(
+        'app.provider_client.get_all_providers',
+        return_value=stub_providers
+    )
+    mock_update_provider = mocker.patch(
+        'app.provider_client.update_provider'
+    )
+
+    client_request.login(platform_admin_user)
+    page = client_request.post(
+        '.edit_sms_provider_ratio',
+        _data=post_data,
+        _follow_redirects=True
+    )
+
+    assert expected_error in page.select_one('.govuk-error-message').text
+    mock_update_provider.assert_not_called()