Just days before the deadline to comply with China's order, the letter from 22 chambers of commerce and trade groups representing the world's major technology suppliers adds to pressure on Beijing to halt the plan following an official protest by Washington.
The order requires manufacturers to pre-install or supply "Green Dam Youth Escort" software with PCs made for sale in China beginning Wednesday.
"The Green Dam mandate raises significant questions of security, privacy, system reliability, the free flow of information and user choice," said the letter dated Friday, a copy of which was provided to The Associated Press. It appealed to Wen to "reconsider implementing the Green Dam requirements."
Such a direct appeal to Chinese leaders is highly unusual. Companies usually avoid commenting publicly on government policy for fear of retaliation.
The letter was signed by leaders of the American, European and Japanese chambers of commerce in China, the U.S. National Association of Manufacturers and trade groups representing the world's major technology suppliers.
The filtering plan "seems to run counter to China's important goal of becoming a vibrant and dynamic information-based society," the letter said.
Chinese authorities say the "Green Dam" system is needed to block access to violent and obscene material. But analysts who have reviewed the program say it also contains code to filter out material the government considers politically objectionable.
The system also has been criticized by free-speech advocates and some of China's 298 million Internet users.
PC makers received the order in May, giving them little time to test the software and work out how to comply. Manufacturers say they still are trying to obtain details of the plan.
A spokeswoman for China's Ministry of Commerce said she had not heard about the letter and declined to comment. A spokeswoman for the Ministry of Foreign Affairs said it was not in charge of the issue.
Top U.S. trade officials have called on Beijing to revoke the order requiring the filtering software, calling it a "serious barrier to trade" and citing security concerns. They said Beijing might have violated its World Trade Organization commitments by failing to give companies adequate advance notice and time to comment.
