diff --git a/pydantic_xml/utils.py b/pydantic_xml/utils.py
index 0b137b3..0553e6a 100644
--- a/pydantic_xml/utils.py
+++ b/pydantic_xml/utils.py
@@ -81,7 +81,7 @@ def register_nsmap(nsmap: NsMap) -> None:
     """
 
     for prefix, uri in nsmap.items():
-        if prefix != '':  # skip default namespace
+        if prefix != '' and not re.match(r"ns\d+$", prefix):  # skip default namespace and reserved ones
             etree.register_namespace(prefix, uri)