China’s largest social media platforms, Weibo, WeChat and Baidu Tieba, are under investigation for alleged violations of cyber security laws.
The Office for Cyberspace Administration said the three platforms had failed to police content on their sites.
It said people had been using the platforms to spread terrorism-related material, rumours and obscenities.
Weibo, WeChat and Baidu’s Tieba are among the most powerful social media platforms in the world, each attracting hundreds of millions of users in China.
The move to place the three platforms under investigation will almost certainly prompt the sites’ owners to do even more to police their own content.
The authorities in China heavily censor the internet, routinely blocking content or search terms and removing posts they consider sensitive.
They also block foreign social media sites and apps, including Facebook, Instagram and Twitter.
Search engines like Google are blocked, and access to many foreign media outlets is restricted too.
It is also clamping down on users who try to get around the restrictions, by tightening regulations on Virtual Private Networks.