Hong Kong, Aug 2 (Reuters) - China's cyberspace regulator said on Wednesday children under the age of 18 should be limited to a maximum of two hours a day on their smartphones, sending shares in tech companies tumbling.

The Cyberspace Administration of China (CAC) said it wanted providers of smart devices to introduce so-called minor mode programmes that would bar users under 18 from accessing the internet on mobile devices from 10 p.m. to 6 a.m.

Providers would also have to set time limits under the proposed reforms, the CAC said.

Users aged 16 to 18 would be allowed two hours a day, children aged eight to 16 would get one hour while children under eight would be allowed just eight minutes.

But the CAC said service providers should allow parents to opt out of the time limits for their youngsters.

Investors were not impressed.

Shares in Chinese tech firms mostly fell in afternoon trade in Hong Kong after the CAC published its draft guidelines, which it said were open to public feedback until Sept. 2.

Bilibili (9626.HK) and Kuaishou (1024.HK) slid 6.98% and 3.53% respectively while Tencent Holdings (0700.HK) , which operates the social network app WeChat, closed 2.99% lower.

Xia Hailong, a lawyer at the Shanghai Shenlun law firm, said the rules would be a headache for the internet companies.

“A lot of effort and additional costs to properly implement these new regulatory requirements," he said.

"And the risk of non-compliance will also be very high. So I believe that many internet companies may consider directly prohibiting minors from using their services."

Authorities have in recent years grown increasingly concerned about rates of myopia and internet addiction among young people.

In 2021, the government imposed a curfew for video game players under the age of 18. That dealt a huge blow to gaming giants like Tencent.

Video-sharing platforms like Bilibili, Kuaishou and ByteDance have since 2019 offered "teenage modes" that restrict the users' access to content and the duration of use.

ByteDance's TikTok-like app Douyin bars teenagers from using it for more than 40 minutes.

The proposed rules come after signals from Beijing that a years-long regulatory crackdown on its technology industry has ended. Authorities have said they will look to support the development of tech giants.