Threats range from governments trying to control citizens to the rise of Facebook and Apple-style "walled gardens", says tech billionaire.
The principles of openness and universal access that underpinned the creation of the internet three decades ago are under greater threat than ever, according to Google co-founder Sergey Brin.
In an interview with the Guardian, Brin warned that there were "very powerful forces that have lined up against the open internet on all sides and around the world. I am more worried than I have been in the past . . . it's scary."
He said the threat to the freedom of the internet came from a combination of governments increasingly trying to control access and communication by their citizens, the entertainment industry attempting to crack down on piracy, and the rise of "restrictive" so-called walled gardens such as Facebook and Apple, which tightly controlled what software could be released on their platforms.
Eyes on the future ... Google co-founder Sergey Brin tries a pair of the company's internet-connected glasses at a function in San Francisco earlier this month.
The 38-year-old billionaire, whose family fled anti-semitism in the Soviet Union, was widely regarded as having been the driving force behind Google's partial pullout from China in 2010 over concerns about censorship and cyber-attacks.
He said five years ago he did not believe China or any country could effectively restrict the internet for long but he had been proven wrong: "I thought there was no way to put the genie back in the bottle, but now it seems in certain areas the genie has been put back in the bottle."
Although he said he was most concerned by the efforts of countries such as China, Saudi Arabia and Iran to censor and restrict use of the internet, he also warned that the rise of Facebook and Apple, which have their own proprietary platforms and control access to their users, risked stifling innovation and Balkanising the web.
"There's a lot to be lost," he said. "For example all the information in apps - that data is not crawlable by web crawlers. You can't search it."
Brin's criticism of Facebook is likely to be seen as controversial with the social network approaching an estimated $US100 billion flotation. Google's upstart rival has seen explosive growth, with more than 800 million members worldwide and one in two of all Americans with computer access signed up.
Sign up for CIO Asia eNewsletters.