A small US firm specialising in parental controls for computers said recently that bits of its code were pirated in internet filtering software mandated for computers sold in China.

Solid Oak Software in Southern California has asked US computer makers not to install the Green Dam Youth Escort internet filtering program on machines for the China market and is looking into its legal or diplomatic options.

"It is just an out and out case of piracy," said Solid Oak founder and president Brian Milburn.

"Our product is designed as a tool for parents to keep their children out of places on the internet where they shouldn't be. It's not a tool for filtering an entire continent; we don't believe in that kind of thing at all."

Solid Oak has long made "CyberSitter" parental control software that parents can install on computers to limit what their children can do on the internet.

Portions of CyberSitter code that list which websites to watch for and what to do after finding them were replicated exactly in filtering software that China is mandating for machines sold in that country, Milburn said.

"The actual files they were distributing with the Green Dam software were typed by us; they had our name on them and contained comments we put in our code," Milburn said. "It was absolutely blatant. There is no mistaking it."

Green Dam maker Jinhui Computer System Engineering Inc in China has publicly denied doing anything wrong.

China has told computer makers it wants Green Dam pre-installed on machines heading for that market as of July.

Solid Oak has sent major computer makers a "nice" cease-and-desist letter asking them not to install Green Dam until piracy concerns are addressed, according to Milburn.

"We are hoping they will be cooperative and we will be able to figure this out," Milburn said of computer makers.

"We've received offers from groups in China to help us for free. Mandatory filters aren't a real popular idea for democracy fans in China."