Twitter has pulled out of the European Union's voluntary code to fight disinformation, the EU has said.
Thierry Breton, who is the EU's internal market commissioner, announced the news on Twitter - but warned the firm new laws would force compliance.
"Obligations remain. You can run but you can't hide," he said.
Twitter will be legally required to fight disinformation in the EU from 25 August, he said, adding: "Our teams will be ready for enforcement."
Twitter has not confirmed its stance on the code or responded to a request for comment.
Dozens of tech firms both big and small signed up to the EU's disinformation code, including Meta - which owns Facebook and Instagram - as well as TikTok, Google, Microsoft and Twitch.
The code was launched in June last year, and aims to prevent profiteering from disinformation and fake news, as well as increasing transparency and curbing the spread of bots and fake accounts.
Firms that sign the code can decide which pledges to make, such as co-operating with fact-checkers or tracking political advertising.
Under Elon Musk's ownership, moderation at Twitter has reportedly been greatly reduced - which critics say has allowed an increase in the spread of disinformation.
The social media giant used to have a dedicated team that worked to combat co-ordinated disinformation campaigns, but experts and former Twitter employees say the majority of these specialists resigned or were laid off.
Last month, the BBC found hundreds of Russian and Chinese state propaganda accounts were thriving on Twitter.
But Twitter boss Musk claimed there was now "less misinformation rather than more" since he took over last October.
Alongside the voluntary code, the EU has also brought in a Digital Services Act - a law which obliges firms to do more to tackle illegal online content.
From 25 August, platforms with more than 45 million monthly active users in the EU - which includes Twitter - will have to comply legally with the rules under the DSA.
The law will mean Twitter will have to have a mechanism for users to flag illegal content, act upon notifications "expeditiously" and put in measures to address the spread of disinformation.
On Friday, AFP news agency quoted a European Commission official as saying: "If (Elon Musk) doesn't take the code seriously, then it's better that he quits."