Linux vendors will be unable to license Microsoft's interoperability patents under the terms that were mandated by the European Commission, open source legal experts argue.

It is claimed that the the terms are incompatible with the General Public Licence (GPL), the licence that governs the Linux operating system.

"The agreement is going to run foul of the GPL," Mark Webbink, a director with the Software Freedom Law Center told vnunet.com. The group offers legal support to open source developers and users.

Full details of the agreement have yet to be published, but Webbink bases his assertions on information that has been released so far.

Siding with Webbink, Red Hat's general counsel Michael Cunningham told vnunet.com in an emailed statement that the firm has "concerns that the patent arrangements may have not been made compatible with open source licensing".

A spokesperson for the European Union did not immediately return a request for comment. In a fequently asked questions section about the agreement, the EU cautioned that "some [open source] licences are incompatible with the patent licence offered by Microsoft," but doesn't mention which ones.

Microsoft on Monday agreed to end its appeal against a 2004 European Commission ruling that seeks to ensure fair competition in the software market. The ruling mandates that Microsoft provides third party software developers with the interoperability information required to create products that work well with Microsoft software.

Microsoft will offer non-public interoperability information at a €10,000 flat fee. Commercial vendors can purchase a licence for Microsoft's patents on the interoperability information at a royalty fee of 0.4 per cent of revenues, while non-commercial open source projects will receive a royalty free patent pledge.

The EU stressed that the agreement would open up the interoperability information to open source developers. "I have always said that open source software developers must be able to take advantage of [the 2004 ruling]: now they can," the Commission's competition commissioner Neelie Kroes boasted in a press release.