Solid Oak Software has sent cease-and-desist orders to Lenovo, Acer, Gateway, Sony and Toshiba, following similar orders sent to Hewlett-Packard and Dell earlier this week, Solid Oak spokeswoman Jenna DiPasquale said in an e-mail Thursday.

The move added pressure over intellectual property theft to concerns that the Web filtering software, called Green Dam Youth Escort, could be used to bolster China's censorship of the Internet.

China last month ordered PC makers to distribute Green Dam with all computers sold in the country after July 1. The program blocks both pornography and some political content, including Web sites that mention Falun Gong, the spiritual movement banned as a cult in China. China has said the program is meant to protect children and can be disabled or uninstalled.

Solid Oak last week found that the Chinese software used code written in the proprietary format used by CyberSitter, the company's online content filter targeted at parents, DiPasquale said. The Chinese program contained blacklists and files apparently obtained from CyberSitter, according to a report by researchers at the University of Michigan.

An update distributed through the Chinese program has since disabled the copied blacklists, but the version available for download online does not yet reflect the changes, the researchers said in an addition to their report yesterday.

No one at the main company that developed Green Dam, Jinhhui Computer System Engineering, was immediately available for comment.

Solid Oak has not yet heard back from the PC makers it contacted, DiPasquale said. Its next steps could include seeking a U.S. court injunction to stop the companies from distributing Green Dam in China, she said.

An HP spokeswoman said the company is seeking more information regarding Green Dam in cooperation with the Information Technology Industry Council (ITI), a U.S. trade group. She confirmed HP had received Solid Oak's cease-and-desist order, but declined to comment further on its response.

Lenovo said it is closely monitoring developments involving Green Dam and will continue to obey the law in the countries where it does business.

Industry groups including the ITI have called on China to reconsider requiring distribution of the software, and Chinese state media yesterday said foreign companies might not be able to comply with the mandate on time.

"All domestic PC makers are ready to include the software by July 1, but some foreign PC makers, such as Dell, might not be able to meet the deadline," the China Daily quoted an unnamed official as saying.