Starmer, who is due to make a speech later on Monday, is said to have decided to proceed with restrictions after speaking to bereaved parents and considering evidence from Australia, which brought in a ban for under-16s last December.
Asked about the report, a source at Downing Street said: "The prime minister is not afraid about taking on the tech companies and their bosses to protect young people."
A source close to the matter said a formal ban was unlikely to come this week.
The government could more immediately give details on efforts to prevent children from producing sexualized images online which can be used for sextortion purposes.
Worries over the impact of social media on mental health and online safety prompted Britain to hold a consultation on children's access to social media earlier this year, with curfews, time limits and curbs on addictive design features, all under consideration.
