TikTok’s endless scroll of irresistible content, tailored for each person’s tastes by a well-honed algorithm, has helped the service become one of the world’s most popular apps.

Now European Union regulators say those same features that made TikTok so successful are most likely illegal.

On Friday, the regulators released a preliminary decision that TikTok’s infinite scroll, auto-play features and recommendation algorithm amount to an “addictive design” that violates European Union laws for online safety. The service poses potential harm to the “physical and mental well-being” of users, including minors and vulnerable adults, the European Commission, the 27-nation bloc’s executive branch, said in a statement.

The findings suggest TikTok must overhaul the core features that made it a global phenomenon, or risk major fines. European officials said it was the first time that a legal standard for social media addictiveness had been applied anywhere in the world.

“TikTok needs to change the basic design of its service,” the European Commission said in a statement.

TikTok said it planned to challenge the findings “through every means available to us.”

“The commission’s preliminary findings present a categorically false and entirely meritless depiction of our platform,” the company said in a statement.