diff --git a/C4/Letters.pm b/C4/Letters.pm
index 273b1fef21..10c7e2ee9a 100644
--- a/C4/Letters.pm
+++ b/C4/Letters.pm
@@ -1569,8 +1569,8 @@ sub _wrap_html {
     $css = qq{<link rel="stylesheet" type="text/css" href="$css">} if $css;
     return <<EOS;
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
-    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
-<html lang="en" xml:lang="en" xmlns="http://www.w3.org/1999/xhtml">
+    "https://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
+<html lang="en" xml:lang="en" xmlns="https://www.w3.org/1999/xhtml">
 <head>
 <title>$title</title>
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />