China has begun to crack down on VPN software, a tool employed by internet users to help circumvent the country’s strict censorship rules.

The app for Virtual Private Network provider ExpressVPN was purged from Apple’s Chinese app store on Friday as it contained “content that is illegal in China.”

In a blog post, the company said that “all major VPN apps” had been removed, though they remain accessible to users with billing addresses outside of China.

“We’re disappointed in this development, as it represents the most drastic measure the Chinese government has taken to block the use of VPNs to date, and we are troubled to see Apple aiding China’s censorship efforts. ExpressVPN strongly condemns these measures, which threaten free speech and civil liberties,” ExpressVPN wrote on its blog.

Domestic apps GreenVPN and Haibei VPN terminated their services this month after receiving a notice from the authorities.

US news wire The Associated Press reported that state-owned ISP China Telecom had informed them that VPNs may be used only for “internal office use,” in order to connect to the firm’s headquarters. The Ministry of Industry and Information Technology has claimed that the crackdown, announced in March, will not affect multinational corporations.

For years, the services had helped users leap the “Great Firewall” to access blocked services such as Facebook and Twitter.

Tom is the editor-in-chief and co-founder of Hong Kong Free Press. He has a BA in Communications & New Media from Leeds University and an MA in Journalism from the University of Hong Kong. He has contributed to the BBC, Euronews, Quartz, Global Post and others.