Starmer said he would introduce sweeping changes to social media regulation to better protect the wellbeing of children when they are online.
"It is clear to me a full ban is the right choice," he said, adding that while it would not be easy, the government had agency in pushing back against the power of big technology companies.
As well as a ban on sites such as TikTok, Snapchat and Instagram, the prime minister said he would take action against gaming and livestreaming services which allow children to talk to strangers.
"Is there a situation in the offline world where you would just let your child pair up with a stranger, an adult that you don't know anything about? No, so we're taking action on that," Starmer said.
Britain has increasingly toughened its approach to tech companies in recent years, urging or forcing them to impose age verification, adapt their algorithms and, most recently, prevent children from circulating nude images taken on mobile phones.
But with a growing awareness of the mental health risks posed by children spending too much time online, Starmer has decided to go further after speaking to parents and considering evidence from Australia, which brought in a ban for under 16s last year.
Starmer, who is likely to face a leadership challenge in the coming weeks, said people rightly expected action.
He hoped the ban would be in force by early next year.
"We hope to pass regulation before Christmas, and therefore to bring the ban into force in the early part of next year, probably about springtime," Starmer said.
Australia was the first country to ban social media for children under 16, blocking them in December from platforms including TikTok, Alphabet's YouTube and Meta's Instagram and Facebook.
Since then a raft of countries have said they are looking to regulate access to social media amid mounting concerns over the impact on children's health and safety.
Britain has consulted teachers, parents and young people on new restrictions, including a possible ban for under-16s, as well as curfews, app time limits and curbs on what the government has described as addictive design features.
It received more than 116,000 responses from parents, industry and young people. More than 83 per cent of parents who responded said risks from social media outweighed benefits, while 90 per cent backed a minimum age of 16 to access social media platforms.
While many parents and politicians back a ban, some psychologists and researchers say there is no proof that it will work.
Jon Crowcroft, a communications systems professor at University of Cambridge, said people supporting social bans are well-meaning but probably misguided, and changes could prevent children from accessing sites they need.
"There is a real risk this will drive some users to worse sites and policing devices is close to impossible technically," Prof Crowcroft said.
"Policing platforms is far easier, if only regulators would bother."
with AP
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