The designers of controversial internet filtering software that China has ordered shipped with all new computers said they were trying to fix security glitches in the program.

"Yes we are trying to fix it. But this is normal. Any software has bugs," Bryan Zhang, head of Jinhui Computer System Engineering said.

The code problems are the latest blow to the plan to include the filtering software with all PCs sold here from July 1, which has been criticised overseas and even in China as a bid at mass censorship and a threat to personal privacy.

The government says the Green Dam Youth Escort software is a vital tool needed to prevent young people from having access to pornographic websites.

Chinese authorities have a history of blocking sites that feature porn or politically unacceptable subjects such as the brutal crackdown on Tiananmen pro-democracy protests in 1989 and the banned spiritual group Falungong.