Companies such as Google that build their business on software such as Linux have a moral imperative to contribute back to the free software community, a prominent open-source advocate said yesterday.

Eben Moglen, a Columbia university professor and chairman of the Software Freedom Law Center, made his comments during a speech at the Open Source Business Conference in San Francisco.

Moglen is putting the finishing touches on what he hopes will be the next software licence adopted by developers of the Linux kernel, the GNU GPL (General Public License) Version 3.

Software that is licensed under the GPL can be freely copied and changed, but anyone who distributes the code must then publicly release their modifications under the same license. This means that companies such as Red Hat and Novell must give back all of their Linux code to the community.

But not so for Google or Yahoo, considered two of the largest Linux users on the planet. Although their web-based software is used by hundreds of millions of people, these companies are service providers, not software distributors. So whatever Linux enhancements they have made can legally remain private.