A detailed study of Internet censorship in China and Iran shows that blocking techniques are changing rapidly and are becoming significant new obstacles for news organizations, governments and businesses.

The study, being published on Tuesday, focuses on Internet blocking faced by Iranian and Chinese visitors to BBC Web sites during periods of political unrest in the two countries over the last two years.

“This problem of Internet control is becoming an issue for more than human rights concerns,” said an author of the report, Ronald Deibert, the director of the Canada Center for Global Security Studies and the Citizen Lab, a University of Toronto organization that focuses on Internet security. “The fact is that you have dozens of countries not just filtering for porn, but political filtering and key events as well.”

The study, by the BBC and Dr. Deibert’s center, acknowledges that the Internet accounts for only about 13 percent of the broadcaster’s global audience, which totaled 225 million people in 2010-11. But it is increasingly important in authoritarian countries; for example, there are now 500 million Chinese Internet users, many times the number that listen to shortwave broadcasts.