China has begun a crackdown on the use of virtual private networks, or VPNs, making it harder for internet users to circumvent the Great Firewall.

The nation’s ministry of industry and information technology announced a 14-month “cleanup” of internet access services, including making it illegal to operate a local VPN service without government approval.

VPN services use encryption to disguise internet traffic so that web surfers in China can access websites that are usually restricted or censored by the Great Firewall.

The Great Firewall is a vast internet surveillance and content-control system that prevents people in China from accessing certain websites and pages. It blocks content that’s critical of the Chinese government or that covers controversial political events, such as the Tiananmen Square protest or spiritual movements such as Falun Gong.

Some 171 out of the world’s 1,000 top websites are blocked, including Google, Facebook and Twitter, according to censorship monitor Greatfire.org. In order to access these sites, Chinese internet users can use VPN services, although the government has been trying to clamp down on these for several years.

According to the ministry, the crackdown is designed to “strengthen cyberspace information security management”.

“China’s internet connection service market,” the ministry said, “has signs of ­disordered development that ­require urgent regulation and governance.”

The campaign is timed ahead of a leadership overhaul that takes place at the Communist party congress in late 2017.

Greatfire.org’s Charlie Smith said that the measures predominantly affect domestic players.

“Foreign ones will be largely unaffected unless they offer their customers a China server, in which case they will probably have to register in China or drop the China server,” he said. “Most of them will do the latter.”

Smith is concerned that domestic VPN providers that register with the authorities could share data and information relating to their customers.

“Chinese VPN users know they are accessing information the authorities deem ‘inappropriate’, so potentially they could put themselves in danger if their service provider is working closely with the authorities,” he said.

David Gorodyansky, CEO of Anchor Free, which make the Hotspot Shield VPN, expressed concern that the new measures threaten privacy and security.

VPNs aren’t only used to access restricted content, but also to create secure connections for people or companies transmitting sensitive data, he explained. Companies with an office in China might use a VPN to securely connect to the internet of their US headquarters, for example.

“People and companies use VPNs to protect their privacy when it comes to things like health, wealth and family,” he said. This is because VPNs shield a user’s IP address, making it harder to track a person’s identity or location. They also encrypt the web traffic. “We make everything as secure as your banking site,” he said.

Despite his concerns, Smith thinks the attention around the announcement has created a Streisand Effect that will drive adoption of circumvention technology.

“This story made headlines in the Chinese press, which helps to generally raise awareness about circumvention tools. I would expect that some foreign VPN companies are seeing a slight uptick in new customer signups from China.”