FRANKFURT (Reuters) - A German software group on Wednesday threatened to take U.S. software firm SCO Group to court if it did not withdraw from a worldwide attack on Linux, the free computer operating system.

Linuxtag, a German Linux lobbying association, said it may seek a German court order against SCO this week because of threats made against 1,500 of the world's most important Linux supporters, including International Business Machines Corp.

Linux is considered as the only big rival to Microsoft's Windows and can be found on 15 percent of all computer servers sold in Western Europe. The German government is installing Linux throughout certain ministries.

Hundreds of companies distribute the software, charging little or nothing for the core software, but taking fees on modifications, services and maintenance.

Companies and governments like Linux, written by Linus Torvalds and further developed on the web with the help of thousands of volunteer programmers, because it is a stable software and not controlled by just one company.

But its future as an open system came under threat two months ago when SCO claimed Linux had mixed in some of SCO's proprietary software.

Companies using Linux could be liable to pay license fees to SCO for every computer that runs on Linux, SCO said.

Germany's Linuxtag told SCO to provide proof for its claims by May 30 or face court action in Germany.

"SCO is massively unsettling our members and the companies that are potential exhibitors at the fair with those claims," said Michael Kleinhenz, a spokesman for Linuxtag, which holds a fair under the same name.

"If they don't stop that, or present proof for the intellectual property rights they are claiming, we are going to apply for a preliminary injunction at the court on Friday," Kleinhenze added.

WEAK SPOT FOR LINUX

SCO's German unit said it had received Linuxtag's motion and would discuss with its U.S. parent whether to present proof for the claim before its case against IBM, one of the biggest Linux promoters, comes to court.

"We have this special situation here in Germany with a high affinity for Linux, and a very well-organized Linux community," said Hans Bayer, the head of SCO's German unit. "But I can't go on my own with presenting the proof."

Alongside its copyright court battle with IBM, SCO also notified 1,500 of the world's largest corporations in May that their use of Linux may be in violation of its rights.

Some of the largest German companies have been sent the letter as well, including electronics giant Siemens AG, the German railway system and carmaker Volkswagen, said SCO's Bayer.