The Signal messaging app logo is seen on a smartphone Photograph:( Reuters )
Signal saw a surge in downloads worldwide after January 6 when WhatsApp updated its privacy terms, reserving the right to share user data, including location and phone number, with its parent Facebook Inc and units such as Instagram and Messenger
The encrypted messaging app Signal, recommended by Tesla's Elon Musk, appeared to have been blocked in China on Tuesday, closing off one of the last US social media platforms still freely available in the country.
Users of the free app, which has surged in popularity globally after rising privacy concerns, were unable to access it without a virtual private network (VPN).
China has deployed a vast and sophisticated system to scrub the internet of dissent and prevent citizens from accessing international social networking platforms such as Facebook and Twitter, in what is known as the "Great Firewall".
Signal's website has been banned since at least March 15, according to the GreatFire.org website which tracks online censorship in China, although the app remains available on China's App Store.
The app was already unavailable on third-party Android stores in China, where Google services are largely blocked.
On several occasions, the app had previously stopped working in China without VPN with no reason given, only for usage to resume.
Signal saw a surge in downloads worldwide after January 6 when WhatsApp updated its privacy terms, reserving the right to share user data, including location and phone number, with its parent Facebook Inc and units such as Instagram and Messenger.
Signal had been downloaded close to 510,000 times on iOS in China, and has been downloaded 100 million times worldwide on the App Store and Google Play combined, according to data company Sensor Tower.
The WhatsApp policy update, later delayed, concerned how merchants using WhatsApp to chat with customers can share data with Facebook, which could use the information for targeted ads, according to the social network.
Signal has stopped working without a VPN occasionally before, although service resumed afterwards.
Co-founder of GreatFire.org Martin Johnson said that its data showed that the app was briefly unavailable in the App Store in 2019.
Last month China closed down the Clubhouse app in China, an audio platform which boomed briefly in the country for uncensored discussion of taboo topics.
Tencent's all-in-one mobile app WeChat is China's dominant messaging app, with its payment functions and other services, boasts more than a billion users globally.