The former children's commissioner for England has launched legal proceedings on behalf of 3.5 million children under 13 against TikTok.
Anne Longfield has alleged that the social media platform has illegally collected personal data from millions of children since May 2018 - when General Data Protection Regulation (GDPR) was introduced.
The lawsuit is seeking compensation for millions of potentially affected children, which Ms Longfield said could run into billions of pounds.
The claim argues that TikTok, which was founded by Chinese Company ByteDance, breached data protection rules wilfully, taking children's personal information without warning, transparency or the necessary consent.
It is also alleged that personal data was collected without the knowledge of parents and children.
This is the latest development in the lawsuit against the video-sharing app after the High Court ruled in December that a 12-year-old girl, who was supported by Ms Longfield, could bring the dispute with TikTok anonymously.
TikTok policies in the UK do not allow children under 13 to use the app and those downloading it are asked to input their age when they join.
Figures suggest that many under-13s use the platform.
Ms Longfield said she felt the app's data collection policies, in general, were "excessive for a video-sharing app" but was most troubled by the "collection of data on an industrial scale without either the kids or the parents realising".
TikTok's data collection policy is listed on its website, but Ms Longfield said she felt its practices were "hidden" and "shady".
"In terms of what they take there are addresses, names, date of birth information, their likes, their interests, who they follow, their habits - all of these - the profiling stuff, but also the exact geolocation, that is very much outside what would be deemed appropriate," she said.
"You shouldn't be doing that when it's kids."
Ms Longfield has accused TikTok of being "deliberately opaque" about who has access to data, but notes the company makes billions from advertising revenue generated by providing user information to advertisers.
A TikTok spokesperson said: "Privacy and safety are top priorities for TikTok and we have robust policies, processes and technologies in place to help protect all users and our teenage users in particular.
"We believe the claims lack merit and intend to vigorously defend the action."
Ms Longfield, who has instructed US litigation specialists Scott+Scott, hopes it would be a "powerful test case" which would be a "wake-up call" for other social media platforms.
She added that she hoped to force TikTok to delete the data and put new measures in place to protect children.
"I'd like to see them acknowledge the problem, stop collecting the illegal data, delete the illegal data they have and put safeguards in place, so they can demonstrate that they're acting responsibly," she said.
"I'd like to see them reassure parents - they have introduced some measures over recent months - great, I'm pleased when people take action, but while this is absolutely at the core of what the business model is, any action won't get to the heart of what needs to be done.
"So I think they need to communicate that to parents, they need to stop doing it, they need to delete it and put measures in place and then look at how they're going to rebuild trust - I think that really is what we're talking about."
TikTok is one of the world's most popular apps - especially among youngsters -- and has around 100 million users in Europe alone.
The COVID-19 pandemic, with many children having online learning at home, has helped cement its success.
In January, TikTok tightened privacy rules to protect under-16s, with any accounts for those under the age of 16 changed to private.