China's notorious "Great Firewall" is being blamed for widespread reports of virtual private network (VPN) outages in that country.

China's Global Times – an English-language subsidiary of the state-run People's Daily – reports that customers of several prominent VPN services based outside the Middle Kingdom have complained that they have been unable to access the services from the mainland.

"We are currently working diligently to find a resolution with certain servers not working in China," VPN firm StrongVPN said in a statement. "We would like to remind our users, during this period there may be exceptionally high wait times in our Live Chat system."

According to the Global Times, the outages are the result of "upgrade work" being done with the country's massive internet censorship platform. With the new measures in place, access to many VPN services from within China has now been blocked.

The state news agency goes on to quote Chinese security experts as stating that the blocks were important for maintaining "cyberspace sovereignty".

"For instance, a shortcut has to be blocked since it could be used for some ulterior purposes although it might affect others who use it in a right way," Qin An of the China Institute for Innovation and Development Strategy, a government policy think tank, told Global Times.

Overseas VPNs are one way to work around the Great Firewall's strict bans on content. Many Chinese use the services to connect with popular – but locally censored – sites such as Facebook and Twitter. While the government does allow limited VPN use, however, VPN services that wish to operate within China are required to register with the Ministry of Industry and Information Technology for permission.

This isn't the first time China has moved to crack down use of unauthorized VPNs. Back in 2012, a similar measure was taken by authorities to prevent users from connecting to foreign-run services. That time, VPN companies and their customers were able to adapt to the Great Firewall changes with varying degrees of success. ®