WASHINGTON -- In an unusual move that critics contend could disrupt millions of Web sites, the U.S. Patent and Trademark Office is reconsidering a patent affecting Internet pages.

Citing "a substantial outcry from a widespread segment of the affected industry," deputy patent commissioner Stephen Kunin ordered the agency's examiners to reconsider the patent they awarded in November, 1998, to three researchers at the University of California. Mr. Kunin described the case as "an extraordinary situation." The patent office has ordered such re-examinations only 151 times since 1981 and issues about 180,000 patents each year.

The patent affects how Internet sites build into Web pages small interactive programs that power everything from banner ads to interactive customer service. Eolas Technologies Inc., which was founded by one of the inventors and has licensed the patent exclusively, has begun enforcing its claims and recently won a $520-million (U.S.) jury award against Microsoft Corp., which quickly appealed the judgment.

Eolas said Microsoft's Internet Explorer Web browser software -- used by the vast majority of Internet users -- violated its patent. Microsoft has pledged to redesign its browser early in 2004.

Mr. Kunin's Oct. 30 order was based on claims by World Wide Web inventor Tim Berners-Lee and others that patent examiners may not have adequately considered so-called "prior art" that suggested the researchers' ideas were not new.