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

Jinhui Computer System Engineering, 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 websites, too, including those based on keywords rather than specific web addresses.

Parents can also add their own sites to the blocking list, Zhang Chenmin, general manager of Jinhui, said.

"If a father doesn't want his son to be exposed to content related to basketball or drugs, he can block all websites related to those things," Zhang said.

He said users could disable blocking of any site on the list or even uninstall the software completely, but they will not be able to see the full database. He said the software does 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 the nation's internet service providers, the government routinely blocks political sites, especially ones it considers socially destabilising such as sites that challenge the ruling Communist Party, promote democratic reform or advocate independence for Tibet.

The government also bans internet pornography and this year launched a nationwide crackdown that led to the shuttering of more than 1,900 websites. Websites including Google and Baidu, China's most popular search engine, also have been criticised 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.

Although users can unblock sites or uninstall the software, many won't bother or know how, Palfrey said. There's also the possibility of the software leaving traces, he said, giving users a false sense of security if the software blocks or monitors usage anyhow - or giving users enough uncertainty that they'll practice self-censorship.

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

And though the software isn't currently 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."

"Blocking access to pornography sounds like an acceptable goal, but the problem is that it's all too easy to use the same technology to expand the censorship," the group's president, Ed Black, said in a statement.

Zhang said his company, based in Henan capital of Zhengzhou in central China, signed a 21 million yuan (NZ$4.93 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, which declined to comment.

According to The Wall Street Journal, China's Ministry of Industry and Information Technology sent computer makers a notice on 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, adding that 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 by phone or fax.

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

Educators "should fully realise the damage that harmful online information does to the physical and mental health of primary and secondary school students," the notice said.

In a statement, Palo Alto, California-based Hewlett-Packard said it is working with the Information Technology Industry Council, a trade group, to "seek additional information, clarify open questions and monitor developments on this matter."

The trade group declined official comment. PC makers Lenovo Group of Beijing, Dell and Apple did not immediately respond to email and phone requests for comments.