XSLT works in VS but not in ASP.NET runtime - fails with 'Cannot find script'

Question

I am using xslt functions from MSXLT

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl">
...
<xsl:value-of select="msxsl:format-date(/article/createDate, 'dd/MM')"/>

This works when debugging in VS, but when I run it in a web app, it fails: 'cannot find the script or external object that implements prefix msxsl'. Am I missing a reference in my web project?

Thanks for any help

Ryan