Internet censorship is growing worldwide, with 26 out of 40 countries blocking or filtering political or social content, a study reported Friday.

The survey carried out by experts at four leading universities found that people in Asia, the Middle East and North Africa were often denied access to information about politics, sexuality, culture or religion.

Conducting the first of what is planned to become an annual survey, the experts at the universities of Oxford, Cambridge, Harvard and Toronto found that the approach varied according to the country.

For example, South Korea heavily censored only one topic, North Korea, while Iran, China and Saudi Arabia blocked both a wide range of topics and a great deal of content related to those topics.

The experts with the OpenNet Initiative, who carried out their research last year, listed six countries as "pervasive" filterers of political information: Myanmar, China, Iran, Syria, Tunisia and Vietnam.

They categorized seven countries, all of them Muslim, as "pervasive" social filterers: Iran, Oman, Saudi Arabia, Sudan, Tunisia, the United Arab Emirates and Yemen.

Topics blocked are those considered antithetical to social norms, such as pornography, gay and lesbian content, and gambling.

Social filtering also was carried out by countries like France and Germany, where websites that deny the Holocaust or promote Nazism are blocked.

The survey found that Myanmar, China, Iran, Pakistan and South Korea have the "most encompassing national security filtering," targeting the websites of insurgents, extremists, and terrorists.

"The survey shows us that online censorship is growing around the world," said John Palfrey, executive director of the Berkman Center for Internet and Society, and clinical professor of law at Harvard Law School.

"Some regulation is to be expected as the medium matures, but filtering and surveillance can seriously erode civil liberties and privacy and stifle global communications," he added in a statement.

However, the survey found that a handful of countries where Internet filtering might be expected -- such as Afghanistan, Egypt, Iraq, Israel, Malaysia, Nepal, Russia, Venezuela and Zimbabwe -- were found not to filter.

The survey said that Internet filtering techniques have evolved with the growing complexity of content.

"Instead of just blocking static Web sites, such as pages online that show pornographic pictures or information about human rights, online censors are blocking entire applications, such as YouTube," it added.

Other applications that are often targeted are internet telephony service Skype and Google Maps. Still others are blogs, political parties and local non-government organisations.

"In the case of blogs, a number of countries, including Pakistan and Ethiopia, have blocked entire blogging domains," it said.

The survey said the United States and European countries did not come in for testing, as the filtering practices were better understood than in other parts of the world.

The survey marked "the first step towards a comprehensive global assessment of Internet filtering practices," said Oxford University professor Jonathan Zittrain,who expects to find more countries that filter the Internet as testing is expanded.