China appears to be tightening its control of internet services that are able to burrow secretly through what is known as the "Great Firewall", which prevents citizens there from reading some overseas content.

Both companies and individuals are being hit by the new technology deployed by the Chinese government to control what people read inside the country.

A number of companies providing "virtual private network" (VPN) services to users in China say the new system is able to "learn, discover and block" the encrypted communications methods used by a number of different VPN systems.

China Unicom, one of the biggest telecoms providers in the country, is now killing connections where a VPN is detected, according to one company with a number of users in China.

VPNs encrypt internet communications between two points so that even if the data being passed is tapped, it cannot be read. A VPN connection from inside China to outside it also mean that the user's internet connection effectively starts outside the "Great Firewall" – in theory giving access to the vast range of information and sites that the Chinese government blocks. That includes many western newspaper sites as well as resources such as Twitter, Facebook and Google.

Users in China suspected in May 2011 that the government there was trying to disrupt VPN use, and now VPN providers have begun to notice the effects.

Astrill, a VPN provider for users inside and outside China, has emailed its users to warn them that the "Great Firewall" system is blocking at least four of the common protocols used by VPNs, which means that they don't function. "This GFW update makes a lot of harm to business in China," the email says. "We believe [the] China censorship minister is a smart man … and this blockage will be removed and things will go back to normal."

But the company added that trying to stay ahead of the censors is a "cat-and-mouse game" – although it is working on a new system that it hopes will let it stay ahead of the detection system.