I cannot create an XML element with a namespace

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I have got to create an XML file using XMLDocument() like this:

<?xml version="1.0" encoding="UTF-8" ?>
- <xml xmlns:xi="http://www.dpr.co.uk/mplcvalidationtables/">
- <xi:table ValidationTable="3" Heading="A/C Control Status"
Description="Description">
<item Code="AA" CodeDescription="Arrears Arrangement" />
<item Code="AD" CodeDescription="Arrears Arrangement Defaulted" />
<item Code="D" CodeDescription="Deceased" />
</ xi:table>
- < xi:table ValidationTable="4" Heading="Capital Adequacy Grp"
Description="Group Description">
<item Code="0" CodeDescription="Capital Adequacy Group 0" />
<item Code="1" CodeDescription="Adequacy Group 1" />
</ xi:table>

I got mostly all this, but I cannot create an XML element with a namespace
(xi:table), the prefix property seems not to have any effect. And the element
created it’s only <table………………>
Thanks,
Filippo