BEIJING - China is requiring that personal computers sold in the country carry software that blocks online pornography and other Web sites, potentially giving one of the world's most sophisticated censorship regimes even more control over the Internet.

The tool will give parents more oversight by preventing computers from accessing sites with pornographic pictures or language, the software's developer said yesterday.

Jinhui Computer System Engineering Co., which won a government contract to develop the "Green Dam-Youth Escort" filtering software, was compiling a database of sites to block.

Although porn sites are initially targeted, the software could be used to block other Web sites, too, including those based on keywords rather than specific Web addresses.

Parents can also add their own sites to the blocking list, said Zhang Chenmin, general manager of Jinhui. "If a father doesn't want his son to be exposed to content related to basketball or drugs, he can block all Web sites related to those things," he said.

Users could disable blocking of any site on the list or even uninstall the software completely, he said, but they will not be able to see the full database. He said the software did not monitor or send data to third parties.

China, which has the world's largest population of Internet users at more than 250 million, also has one of the world's tightest controls over the Internet.

Through such mechanisms as network-level filters installed at China's Internet service providers, the government routinely blocks political sites, especially ones it considers socially destabilizing such as those that challenge the ruling Communist Party, promote democratic reform, or advocate independence for Tibet.

The government also bans Internet pornography and this year began a nationwide crackdown that led to the shuttering of more than 1,900 Web sites. Sites including Google and Baidu, China's most popular search engine, also have been criticized for linking to suspect sites.

John Palfrey, an Internet censorship expert at Harvard University, described the latest requirements as "a potential game changer in the story of Internet control," by moving China's "Great Firewall" closer to the user, where censorship can be more effective.

"One of the most effective parts of China's controls is self-censorship, the perception that you are being watched or blocked," Palfrey said from Washington.

And though the software is not designed for monitoring usage, Palfrey said a future update could give it surveillance capabilities, something easier to implement once the basic software is already on PCs.

A Washington-based industry trade group, the Computer & Communications Industry Association, denounced China's efforts "to build censorship capabilities right into the hardware."

Zhang said his company, based in Zhengzhou in central China, signed a 21 million yuan ($3 million) contract with the Chinese government last May to develop the software and distribute it to computer-makers for free within one year. The software was jointly developed by Beijing Dazheng Language Technology Co. Ltd., which declined to comment.

According to the Wall Street Journal, China's Ministry of Industry and Information Technology sent computer-makers a notice May 19 that PCs to be sold in China as of July 1 must be preloaded with the software.

The program would either be installed on the hard drive or enclosed on a compact disc, the newspaper reported. It said PC-makers would be required to tell authorities how many PCs they have shipped with the software.

The ministry did not immediately respond to questions from the Associated Press by phone or fax.

A separate notice on its Web site said all primary and secondary schools were required to install the Green Dam software on every school computer by the end of May.