China filtered a wide range of topics, said the report

The study of thousands of websites across 120 Internet Service Providers found 25 of 41 countries surveyed showed evidence of content filtering.

Websites and services such as Skype and Google Maps were blocked, it said.

Such "state-mandated net filtering" was only being carried out in "a couple" of states in 2002, one researcher said.

"In five years we have gone from a couple of states doing state-mandated net filtering to 25," said John Palfrey, at Harvard Law School.

What's regrettable about net filtering is that almost always this is happening in the shadows.

John Palfrey, Harvard Law School

Mr Palfrey, executive director of the Berkman Center for Internet and Society, added: "There has also been an increase in the scale, scope and sophistication of internet filtering."

ONI is made up of research groups at the universities of Toronto, Harvard Law School, Oxford and Cambridge.

It chose 41 countries for the survey in which testing could be done safely and where there was "the most to learn about government online surveillance".

A number of states in Europe and the US were not tested because the private sector rather than the government tends to carry out filtering, it said.

Countries which carry out the broadest range of filtering included Burma, Iran, Pakistan, Saudi Arabia, Syria, Tunisia, the United Arab Emirates and Yemen, the study said.

The filtering had three primary rationales, according to the report: politics and power, security concerns and social norms.

The report said: "In a growing number of states around the world, internet filtering has huge implications for how connected citizens will be to the events unfolding around them, to their own cultures, and to other cultures and shared knowledge around the world."

The report said net censorship was spreading across the globe

Jonathan Zittrain, Professor of Internet Governance and Regulation at Oxford University, said the organisation was also looking at the tools people used to circumvent filtering.

"It's hard to quantify how many people are doing this. As we go forward each year we want to see if some of these circumvention technologies become more like appliances and you just plug them in and they work," he added.

"Few states restrict their activities to one type of content," said Rafal Rohozinski, Research Fellow of the Cambridge Security Programme.

He added: "Once filtering is begun, it is applied to a broad range of content and can be used for expanding government control of cyberspace. It has become a strategic forum of competition between states, as well as between citizens and states."

Mr Palfrey said the report was an attempt to shine a spotlight on filtering to make it more transparent.

"What's regrettable about net filtering is that almost always this is happening in the shadows. There's no place you can get an answer as a citizen from your state about how they are filtering and what is being filtered."

The survey found evidence of filtering in the following countries:

Azerbaijan, Bahrain, Burma/Myanmar, China, Ethiopia, India, Iran, Jordan, Libya, Morocco, Oman, Pakistan, Saudi Arabia, Singapore, South Korea, Sudan, Syria, Tajikistan, Thailand, Tunisia, Turkmenistan, UAE, Uzbekistan, Vietnam and Yemen.