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New rules on the hugely popular TikTok app mean under-16s will no longer be allowed to send or receive direct messages.
It is the first time a major social-media platform has blocked private messaging by teenagers, on a global scale.
A survey by UK regulator Ofcom suggested TikTok was used by 13% of 12- to 15-year-olds last year.
Critics say the new rules will not stop children lying about their age online.
Until now, all users have been able to send direct messages to others, when both accounts follow each other.
The change means those under the age of 16 will no longer be able to communicate privately on the platform under any circumstances.
They will still be able to post publicly in the comments sections of videos.
TikTok says those affected will receive an in-app notification soon and will lose access to direct messages on 30 April.
The limit is based on the date of birth added to the account when it is created - but no verification takes place and the system is based on trust.
In 2018, Facebook introduced rules to make WhatsApp available to over-16s only across the EU, to adhere to its General Data Protection Regulation.