This is the fifth case of the EU executive accusing TikTok of breaching EU rules, even though it has issued no fines against the Chinese-owned platform.

In Friday's announcement, the Commission says the company didn't adequately consider the risk of sending users down "rabbit holes" through a "continuous stream of personalised videos, even when those are related to mental health struggles and body image."

The investigation focused on mental health impacts because "the evidence was strongest," when asked about how these rabbit holes contribute to radicalization, said another official.

The more users interact with content, even if it's potentially harmful, the more of that content they are recommended, the Commission said in its press release, which noted that the measures TikTok has put in place to curb the rabbit hole effect have not been effective.

TikTok should take into account "explicit" signals like buttons that say "I'm not interested" more than implicit ones, like engagement, said the first Commission official. Engagement metrics measure how much users interact with content, such as how much time they spend watching a video or the content they like. Talking about mitigation of these risks, the official said that remedies have to be effective and that TikTok's current measures are not.

"To say 'I'm not interested' has to have a meaning, an effective meaning, and not only should not only be an afterthought, and then you go back to the old recommender system," the official said.