Some of most common VPN (virtual private network) protocols have become unstable ahead of the upcoming military parade, a VPN service provider said Wednesday.

"Due to next week's military parade in Beijing, VPN access from iOS devices may be limited at this time," according to a notice sent by overseas VPN company Astrill to its subscribers.

China is to hold a military parade on September 3 to commemorate the 70th anniversary of the end of World War II

A company technical support staff told a Global Times reporter who posed as a customer on Wednesday that iOS users in China will have limited access in the first week of September due to limitations in the operating system, in which Cisco IPSec is used in its VPN application.

Protocols including Cisco IPSec, L2TP/IPSec and PPTP will be affected, he said, adding that Astrill will be able to provide more resilient VPN protocols for devices running on iOS 9, since Apple said it would make changes to its operating system.

VPN service on routers, PCs and Android devices will not be affected as they use more resilient protocols. 

In January, Astrill announced that some of its users in China have been unable to use its services. VPN protocols used on iOS devices, including IPSec, L2TP/IPSec and PPTP, were not accessible in China, Astrill said in January.

China's Internet authorities said that "improper information" online will be regulated in accordance with Chinese law, amid concerns over the limited availability of overseas VPN services.

Wen Ku, director of the communication development department at the Ministry of Industry and Information Technology, reportedly said in January that "Internet development in China should be in line with the law and some improper information should be regulated."