diff --git a/api/custom_auth/serializers.py b/api/custom_auth/serializers.py
index 03cab2cad5fa..f1128bd980c2 100644
--- a/api/custom_auth/serializers.py
+++ b/api/custom_auth/serializers.py
@@ -6,7 +6,6 @@
 from rest_framework import serializers
 from rest_framework.authtoken.models import Token
 from rest_framework.exceptions import PermissionDenied
-from rest_framework.validators import UniqueValidator
 
 from organisations.invites.models import Invite, InviteLink
 from users.auth_type import AuthType
@@ -68,17 +67,21 @@ class Meta(UserCreateSerializer.Meta):  # type: ignore[misc]
         write_only_fields = ("sign_up_type",)
         extra_kwargs = {
             "email": {
-                "validators": [
-                    UniqueValidator(
-                        queryset=FFAdminUser.objects.all(),
-                        lookup="iexact",
-                        message="Email already exists. Please log in.",
-                    )
-                ]
+                "validators": list[object](),
             }
         }
 
     def validate(self, attrs):  # type: ignore[no-untyped-def]
+        email = attrs.get("email", "")
+        self._validate_registration_invite(
+            email=email, sign_up_type=attrs.get("sign_up_type")
+        )
+
+        if FFAdminUser.objects.filter(email__iexact=email).exists():
+            raise serializers.ValidationError(
+                {"email": "Email already exists. Please log in."}
+            )
+
         attrs = super().validate(attrs)
         email = attrs.get("email")
         if attrs.get("superuser"):
@@ -101,10 +104,6 @@ def validate(self, attrs):  # type: ignore[no-untyped-def]
                 self.context.get("request"), email=email, raise_exception=True
             )
 
-        self._validate_registration_invite(
-            email=email, sign_up_type=attrs.get("sign_up_type")
-        )
-
         attrs["email"] = email.lower()
         return attrs
 
diff --git a/api/tests/integration/custom_auth/end_to_end/test_custom_auth_integration.py b/api/tests/integration/custom_auth/end_to_end/test_custom_auth_integration.py
index 620025537176..ff36358c4b59 100644
--- a/api/tests/integration/custom_auth/end_to_end/test_custom_auth_integration.py
+++ b/api/tests/integration/custom_auth/end_to_end/test_custom_auth_integration.py
@@ -122,6 +122,29 @@ def test_register__without_invite_when_disabled__returns_forbidden(
     assert response.status_code == status.HTTP_403_FORBIDDEN
 
 
+@override_settings(ALLOW_REGISTRATION_WITHOUT_INVITE=False)  # type: ignore[misc]
+def test_register__existing_email_without_invite_when_disabled__returns_forbidden(
+    db: None,
+    api_client: APIClient,
+    admin_user: FFAdminUser,
+) -> None:
+    # Given
+    password = FFAdminUser.objects.make_random_password()
+    register_data = {
+        "email": admin_user.email,
+        "password": password,
+        "first_name": "test",
+        "last_name": "register",
+    }
+
+    # When
+    url = reverse("api-v1:custom_auth:ffadminuser-list")
+    response = api_client.post(url, data=register_data)
+
+    # Then
+    assert response.status_code == status.HTTP_403_FORBIDDEN
+
+
 @override_settings(ALLOW_REGISTRATION_WITHOUT_INVITE=False)  # type: ignore[misc]
 def test_register__with_invite_when_registration_disabled__returns_created(
     db: None,
diff --git a/api/tests/unit/custom_auth/test_unit_custom_auth_serializer.py b/api/tests/unit/custom_auth/test_unit_custom_auth_serializer.py
index fc2f67c9d227..b4c202da442f 100644
--- a/api/tests/unit/custom_auth/test_unit_custom_auth_serializer.py
+++ b/api/tests/unit/custom_auth/test_unit_custom_auth_serializer.py
@@ -132,6 +132,24 @@ def test_invite_link_validation__hash_not_provided__raises_permission_denied(
     assert exc_info.value.detail == USER_REGISTRATION_WITHOUT_INVITE_ERROR_MESSAGE
 
 
+def test_invite_link_validation__existing_email_without_invite__raises_permission_denied(
+    db: None,
+    settings: SettingsWrapper,
+) -> None:
+    # Given
+    settings.ALLOW_REGISTRATION_WITHOUT_INVITE = False
+    FFAdminUser.objects.create(email="testuser@mail.com")
+
+    serializer = CustomUserCreateSerializer(data=user_dict)
+
+    # When
+    with pytest.raises(PermissionDenied) as exc_info:
+        serializer.is_valid(raise_exception=True)
+
+    # Then
+    assert exc_info.value.detail == USER_REGISTRATION_WITHOUT_INVITE_ERROR_MESSAGE
+
+
 def test_invite_link_validation__invalid_hash__raises_permission_denied(
     invite_link: InviteLink,
     settings: SettingsWrapper,