In a severe setback for the University of California system and a private software company, the U.S. Patent and Trademark Office has issued a preliminary ruling that invalidates a patent on Web-browser technology made popular by the Microsoft Corporation.

If the ruling is made final, the university's $520.6-million patent-infringement verdict against Microsoft could be overturned.

The case centers on Web-browser technology that launches and displays plug-ins and applets automatically. The technology is used by Microsoft's Internet Explorer Web browser, as well as by hordes of other Web vendors that launch applications like Macromedia Flash, Java applets, and Adobe's Acrobat Reader.

The University of California owns the patent, which is licensed by Eolas Technologies Inc. Both entities won a patent-infringement suit against Microsoft last August. That verdict prompted the World Wide Web Consortium to ask the patent office in October to reconsider the Web-browser patent. The consortium said the patent was not based on unique designs.

The patent office agreed to the re-examination and issued a preliminary ruling last month.

Tim Berners-Lee, director of the consortium, told the patent office in a letter that the patent harms all Web developers, not just those who allegedly had infringed the patent. Enforcing the patent, he wrote, would threaten years of work that have gone into building the Web.

"Given the interdependence of Web technology," he said, "those who wrote Web pages or developed software in reliance on Web standards will now have to retrofit their systems."

The university will appeal the patent office's ruling, said Trey Davis, a spokesman for the University of California.