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."