Elon Musk has been warned he must protect Twitter users from harmful content after he struck a $44bn deal to buy the social media giant.
The billionaire vowed to relax content restrictions, sparking fears from human rights groups it could lead to a rise in bullying and misinformation.
“Regardless of ownership, all social media platforms must be responsible,” Boris Johnson’s spokesman said.
The European Commission also warned Mr Musk that he must protect users.
“Be it cars or social media, any company operating in Europe needs to comply with our rules – regardless of their shareholding,” Thierry Breton, commissioner for the internal market at the EU executive, warned in a tweet.
The European Union has said its new online rules will “overhaul” the digital market, including how tech giants operate.
Mr Musk, who is a self-described “free speech absolutist”, has been vocal in his criticism of Twitter’s policies on moderating content, arguing that it needs to be a genuine forum for free speech.
“Free speech is the bedrock of a functioning democracy, and Twitter is the digital town square where matters vital to the future of humanity are debated,” he said on announcing the deal.
The social media firm faces growing pressure from politicians and regulators over the content that appears on its platform. It has drawn critics from left and right over its efforts to mediate misinformation.
In one of its most high-profile moves, last year it banned former US President Donald Trump, perhaps its most powerful user, citing the risk of “incitement of violence”.
