diff --git a/doc/go1.16.html b/doc/go1.16.html
index b6df0487ca..1239217eed 100644
--- a/doc/go1.16.html
+++ b/doc/go1.16.html
@@ -302,6 +302,20 @@ Do not send CLs removing the interior tags from such phrases.
   </dd>
 </dl><!-- crypto/x509 -->
 
+<dl id="encoding/xml"><dt><a href="/pkg/encoding/xml/">encoding/xml</a></dt>
+  <dd>
+    <p><!-- CL 264024 -->
+      The encoder has always taken care to avoid using namespace prefixes
+      beginning with <code>xml</code>, which are reserved by the XML
+      specification.
+      Now, following the specification more closely, that check is
+      case-insensitive, so that prefixes beginning
+      with <code>XML</code>, <code>XmL</code>, and so on are also
+      avoided.
+    </p>
+  </dd>
+</dl><!-- encoding/xml -->
+
 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
   <dd>
     <p><!-- CL 233637 -->
diff --git a/src/encoding/xml/marshal.go b/src/encoding/xml/marshal.go
index 0554b0d204..d8a04a95a2 100644
--- a/src/encoding/xml/marshal.go
+++ b/src/encoding/xml/marshal.go
@@ -345,8 +345,11 @@ func (p *printer) createAttrPrefix(url string) string {
 	if prefix == "" || !isName([]byte(prefix)) || strings.Contains(prefix, ":") {
 		prefix = "_"
 	}
-	if strings.HasPrefix(prefix, "xml") {
-		// xmlanything is reserved.
+	// xmlanything is reserved and any variant of it regardless of
+	// case should be matched, so:
+	//    (('X'|'x') ('M'|'m') ('L'|'l'))
+	// See Section 2.3 of https://www.w3.org/TR/REC-xml/
+	if len(prefix) >= 3 && strings.EqualFold(prefix[:3], "xml") {
 		prefix = "_" + prefix
 	}
 	if p.attrNS[prefix] != "" {
diff --git a/src/encoding/xml/marshal_test.go b/src/encoding/xml/marshal_test.go
index 31309ef2ca..d2e5137afd 100644
--- a/src/encoding/xml/marshal_test.go
+++ b/src/encoding/xml/marshal_test.go
@@ -2283,6 +2283,30 @@ var encodeTokenTests = []struct {
 		}},
 	},
 	want: `<foo xmlns="space"><bar xmlns="space" xmlns:space="space" space:attr="value">`,
+}, {
+	desc: "reserved namespace prefix -- all lower case",
+	toks: []Token{
+		StartElement{Name{"", "foo"}, []Attr{
+			{Name{"http://www.w3.org/2001/xmlSchema-instance", "nil"}, "true"},
+		}},
+	},
+	want: `<foo xmlns:_xmlSchema-instance="http://www.w3.org/2001/xmlSchema-instance" _xmlSchema-instance:nil="true">`,
+}, {
+	desc: "reserved namespace prefix -- all upper case",
+	toks: []Token{
+		StartElement{Name{"", "foo"}, []Attr{
+			{Name{"http://www.w3.org/2001/XMLSchema-instance", "nil"}, "true"},
+		}},
+	},
+	want: `<foo xmlns:_XMLSchema-instance="http://www.w3.org/2001/XMLSchema-instance" _XMLSchema-instance:nil="true">`,
+}, {
+	desc: "reserved namespace prefix -- all mixed case",
+	toks: []Token{
+		StartElement{Name{"", "foo"}, []Attr{
+			{Name{"http://www.w3.org/2001/XmLSchema-instance", "nil"}, "true"},
+		}},
+	},
+	want: `<foo xmlns:_XmLSchema-instance="http://www.w3.org/2001/XmLSchema-instance" _XmLSchema-instance:nil="true">`,
 }}
 
 func TestEncodeToken(t *testing.T) {