The sweeping changes will "give kids their childhood back," Starmer told reporters, outlining measures against Snapchat, TikTok, Instagram and other platforms as well as gaming sites that allow strangers to contact children.
"It is clear to me a full ban is the right choice," he said.
"It will make a huge difference, it will make our children safer, it will make our children happier, it will give them more time, more security, more freedom to grow up, more opportunity".
However some experts doubted whether a blanket ban would be effective, and Starmer acknowledged it would be difficult to fully enforce such restrictions.
The United Kingdom will go further than Australia - the first country to ban social media for children - with controls on gaming platforms and the possibility of overnight curfews and curbs on infinite scrolling for under-18s.
YouTube, Facebook and X will be covered, the government said, but messaging services such as WhatsApp and Signal will not be.
"World-leading blocks on livestreaming and strangers contacting children would also be imposed," Starmer said.
"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?" he said.
Services designed for children and education such as YouTube Kids, Lego Play and Google Classroom will not be affected by the ban, he said.
While a majority of parents and politicians back a ban, some psychologists and researchers have said there is no proof that it would work, and a group of school children in London told Reuters they had a conflicted relationship with the technology.
Social media companies have already put in place child safety measures, such as new algorithms, in response to tightening regulations, including by the UK.
They said on Monday that a blanket ban could push young people onto riskier platforms that did not offer the protections they had introduced.
YouTube said it had invested in "expert-led, age-appropriate experiences and default protections for teens for over a decade".
"YouTube is a vital resource for young people, educators and parents," a spokesperson said on Monday.
Snapchat said an outright ban would disconnect teens from private messaging between friends and family that accounted for the majority of time spent on its service, adding that the scope of the ban should be reviewed.
And Meta, owner of Facebook and Instagram, said the experience in Australia suggested that bans risked isolating teens from online communities, driving them to unregulated alternatives that lacked protections and parental controls.
A ban could be in place next northern hemisphere spring, Starmer said, underpinned by existing powers and new regulations due by the end of the year.
The ban will likely require age checks to be expanded to all users, something regulator Ofcom has already introduced for pornography sites.
Ofcom said it was ready to work on that.
Starmer, facing a likely leadership challenge in the coming weeks, acknowledged that children would get around the restrictions but said a ban - which could be his main legacy - would bring long-term change to the culture around social media.
