The European Union announced a formal investigation Monday into X, the social media platform owned by Elon Musk, accusing it of failing to counter illicit content and disinformation, a lack of transparency about advertising and “deceptive” design practices.

The inquiry is perhaps the most substantial regulatory move to date against X since it scaled back its content moderation policies after Musk bought the service, once known as Twitter, last year. The company’s new policies have led to a rise in incendiary content on the platform, according to researchers, causing brands to scale back advertising.

In going after X, the European Union is for the first time using the authority gained after last year’s passage of the Digital Services Act. The law gives regulators vast new powers to force social media companies to police their platforms for hate speech, misinformation and other divisive content.

The European Commission, the 27-nation bloc’s executive branch, had signalled its intention to look more closely at X’s business practices. In October, regulators initiated a preliminary inquiry into the spread of “terrorist and violent content and hate speech” on X after the start of the Israel-Gaza conflict.

“The evidence we have is enough to formally open a proceeding against X,” Margrethe Vestager, the commission’s executive vice president overseeing digital policy, said.