China's ministry of industry and information technology issued a notice to personal computer-makers on 19 May that every machine sold from 1 July must be preloaded with the software – called Green Dam Youth Escort, developed by Jinhui Computer System Engineering Co.

The  software  developer ,  which won a government contract to develop the "Green Dam-Youth Escort" filtering software, also updates a list of forbidden sites  to block  from an online database, much as network security programs automatically download the latest defences against new worms, trojans and viruses.

Last year,  40m PCs were sold in China, the world's second biggest market after the US.

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

Parents can also add their own sites to the blocking list, Zhang Chenmin, general manager of Jinhui, told The Associated Press.

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.

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

"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 ($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.

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.

Lawyers and academics are challenging the legality of the Chinese government's requirement for manufacturers to ship Web-filtering software with all personal computers, amid growing concern that it could be used to censor political content, not just pornography.