By Tom Singleton, Technology reporter, BBC News
X owner Elon Musk speaks during the Atreju political meeting organised by the Italian right wing party Fratelli d'Italia in Rome on 16 December 2023. Photo: AFP
The European Union has formally announced it suspects X, previously known as Twitter, of breaching its rules in areas including countering illegal content and disinformation.
Digital commissioner Thierry Breton set out the alleged infringements in a post on the social media platform.
He said X, which is owned by Elon Musk, was also suspected of breaching its obligations on transparency.
X said it was "co-operating with the regulatory process".
In a statement the firm said it was "important that this process remains free of political influence and follows the law".
"X is focused on creating a safe and inclusive environment for all users on our platform, while protecting freedom of expression, and we will continue to work tirelessly towards this goal," it added.
These are the first formal proceedings launched under the Digital Services Act (DSA), the tough new rules for big tech firms the EU has introduced.
The DSA places extra obligations on major companies to protect users against extreme content. If they fail to do so they can face enormous fines or be suspended.
"Today, we opened formal proceedings against X based on several suspected infringements of the Digital Services Act," EU Commission spokesman Johannes Bahrke said.
"The opening of proceedings means that the Commission will now investigate X's systems and policies related to certain suspected infringements. It does not prejudge the outcome of the investigation."
In October the EU said it was investigating X over the possible spread of terrorist and violent content, and hate speech, after Hamas' attack on Israel.
X said then that it had removed hundreds of Hamas-affiliated accounts from the platform.
Explaining the latest steps in its investigation into X on Monday, the EU said its probe would also consider the effectiveness of X's so-called Community Notes system.
It allows contributors to comment on the accuracy of posts, with the company considering it a bulwark against disinformation.
However, concerns about the nature of the content appearing on X have intensified since it was bought by Elon Musk - in part because he laid off many of its moderators - with the European Commission previously warning it had the biggest disinformation problem of any major platform.
In the US, controversy over extremist material appearing on the site has led to an advertising boycott, a bitter row between Musk and a campaign group, and even questions about whether X could end up going bankrupt.
- This story was first published by the BBC