Brussels has warned TikTok that its endlessly scrolling feeds may breach Europe’s new content rules, as regulators press ahead with efforts to rein in the social effects of big online platforms. In preliminary findings issued on Friday, the European Commission said it believed the group had failed to adequately assess and mitigate the risks posed by addictive design features that could harm users’ physical and mental wellbeing, particularly children and other vulnerable groups. The warning marks one of the most advanced tests yet of the EU’s Digital Services Act, which requires large online platforms to identify and curb systemic risks linked to their products. By constantly “rewarding” users with new content, TikTok’s design fuels the urge to keep scrolling and shift the brain of users into “autopilot mode,” the Commission said. “Social media addiction can have detrimental effects on the developing minds of children and teens,” said Henna Virkkunen, the EU’s tech chief. “In Europe, we enforce our legislation to protect our children and our citizens online.” If the provisional conclusions are confirmed, the social media platform could be fined up to 6 percent of its global turnover.