A lawyer for the Free Software Foundation (FSF), part of the open source GNU Project, has described as "moonshine" SCO's claim that the GNU General Public Licence (GPL) is invalid.

Eben Moglen, professor of law at Columbia University, and a lawyer for the FSF, said that SCO's argument would, by extension, render Microsoft's method of distributing Windows invalid as well.

The GNU licence ensures that open source software, including Linux, can be distributed and modified freely and forbids anyone to deny these rights.

SCO's lawyer, Mark Heise of Boies, Schiller and Flexner, claimed last week that the GPL was "pre-empted" by US federal copyright law which, he said, only allowed one copy of software to be made.

SCO has brought a $3bn lawsuit against IBM, claiming that it had misappropriated SCO's code by allowing it to be incorporated into Linux. But IBM's countersuit claims that SCO violated the GPL.

But Moglen said: "Of course, Heise's statement is nothing but moonshine, based on an intentional misreading of the [US] Copyright Act that would fail any law school copyright examination."

Moglen argues that, if it were true that only one copy could be made, no copyright licence could permit the licensee to make multiple copies.

This would not only invalidate all other free software licences, but would eliminate Microsoft's method of distributing Windows.

"It would invalidate the Microsoft Shared Source Licence," said Moglen, adding that Windows was "pre-loaded by hard drive manufacturers onto disk drives they deliver by the hundreds of thousands to PC manufacturers".

"The GPL protects against the baseless claims made by SCO for licence fees to be paid by users of free software, and also prohibits SCO from its ongoing distribution of the Linux kernel, a distribution which infringes the copyrights of thousands of contributors to the kernel throughout the world," said Moglen.

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