The OpenNet Initiative found that 25 countries out of 41 surveyed are blocking websites for political or social reasons, compared to only a few states that restricted web access in 2002.
According to the study, "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."
It identified China, Iran, Myanmar, Syria, Tunisia, and Vietnam as particularly restrictive in censoring political websites. The study added that several Middle Eastern countries had the most extensive filters for websites that offer pornography or gambling, and gay and lesbian sites.
Azerbaijan, Tajikistan, Turkmenistan, and Uzbekistan were also named among the countries where governments interfere in citizens' access to the Internet.
The study found no website filtering in Russia, Israel, or the Palestinian territories.
The OpenNet Initiative is a collaboration between research institutes at four universities in Canada, Britain, and the United States.