Computer makers in China have been instructed to pre-install blocking software on every PC hard drive from next month, under a government push to control access to the internet.
The new software, which has been developed by companies working with the Chinese military, is specifically aimed at restricting online pornography, but it could also be used to strengthen barriers to politically sensitive websites.
China's authorities currently block overseas-based sites they disapprove of, such as those relating to Tibetan independence, or the Falun Gong spiritual movement, with a mesh of filters and keyword restrictions, widely known as the Great Firewall.
Control over domestic servers is applied through instructions to content providers and search engines, which must self-censor to stay in business.
The new software – called Green Dam Youth Escort – potentially adds a powerful new tool at the level of the individual computer. It updates a list of forbidden sites from an online database, much as network security programs automatically download the latest defences against new worms, trojans and viruses.
The software, designed to work with the Microsoft Windows operating system, also collects private user data.
Optional programs that allow parents to restrict internet access by their children have existed for some time, but this is the first time the government has instructed that every computer be installed with a single centralised system.
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. Last year 40m PCs were sold in China, the world's second biggest market after the US.
According to the Wall Street Journal, the notice says the move aims to "construct a green, healthy and harmonious internet environment, and prevent harmful information on the internet from influencing and poisoning young people".
PC makers and the providers of Green Dam are required to report periodically to the ministry about sales figures and the number of copies installed. For at least the first year the software will be free.
A separate instruction on the ministry's website obliged schools to install Green Dam on every computer in their institutions by the end of last month.
The software was developed by Jinhui Computer System Engineering in Henan under a 21m yuan (£2.2m) deal with the government.
Bryan Zhang, the founder of Jinhui, told reporters his company was compiling a database of forbidden sites, all related to pornography. He claimed users would have the option of uninstalling the software, or choosing to unblock sites, though they will not be permitted to see the list.
According to the company website, Jinhui has long-term working relationships with the ministry of public security and the People's Liberation Army's Information Engineering University.
The other collaborator on the project, Beijing Dazheng Human Language Technology Academy, works with the Armoured Engineering Institute of the People's Liberation Army.
China periodically launches campaigns against online porn. In the latest drive more than 1,900 websites have been shut down and search engines, such as Google and Baidu, have been castigated for failing to self-regulate. Rights groups say the same techniques, along with cruder methods, are used to stifle websites that embarrass, irritate or threaten the government.
Last week the authorities blocked Twitter and Hotmail in the run-up to the 20th anniversary of the Tiananmen Square crackdown.
