Tech companies such as Apple and Google have been asked by the UK government to block access to naked images on smartphones and other devices for under-18s.
Sir Keir Starmer has told firms to either activate built-in features or update operating systems to prevent children from taking, sending or viewing sexually explicit images on their phones and other devices.
Speaking at London Tech Week, the prime minister said: "This is not an impossible challenge. These are some of the most innovative companies in the world and I believe they can solve it."
The government said it will bring forward legislation to force firms to activate the features if they do not comply voluntarily within three months.
This could include fines or, as a last resort, criminal liability for companies which do not comply.
The prime minister said the changes would apply to both existing and newly-sold smartphones and tablets in the UK.
"Legislation could cover operating system providers and others in the supply chain, such as retailers, and will not affect the use of devices owned and used by adults who verify their age," he said.
In response, a Google spokesperson said it was "deeply committed to protecting children online."
"We are working constructively with UK partners to find effective, privacy-preserving solutions that deter the spread of harmful content while ensuring a safe digital environment for young people."
Apple has already age-verified its UK users and offers a blocking service on its own platforms including iMessage.
The UK has already introduced laws to try to protect children online.
Foremost is the Online Safety Act - laws and duties online platforms must follow - implemented and enforced by media regulator Ofcom.
However, child safety campaigners have urged the government to go further to stop children seeing or sharing nude images, amid concerns about online grooming and sextortion.
The government said 91% of online child sexual abuse reports recorded in 2024 contained self-generated content from children themselves and the average child now views pornography by the age of 13.
The news comes as Starmer is believed to be preparing to announce a crackdown on children's access to social media within days, including a speech on potential policy expected next week.
Home Secretary Shabana Mahmood said tech companies "have a moral duty to act by making it impossible for children to take, share or view nude images. If they don't, we will legislate".
Technology Secretary Liz Kendall said: "Companies should switch these protections on by default, for every child, on every device. We are giving them three months to show us that they will do the right thing."
Andy Burrows, chief executive of the Molly Rose Foundation, welcomed what he said was "an important step forward for child protection".
However, he criticised government communication around the announcement, alongside one of a potential ban on social media for young people.
"Number 10 needs to start urgently listening to experts and the evidence, rather than rush out hurried announcements for short-term expediency that will quickly unravel and continue to let tech firms off the hook for preventable harm," he said.
Following a national consultation, the UK is moving closer to restrictions on under-16s' social media use, with ministers consulting on measures ranging from age limits to bans on addictive platform features.
Silkie Carlo, director of Big Brother Watch, said: "Protecting children online is vital, but these are outrageous plans that will fail to address the underlying causes of online harm."
"Put simply, the Labour government is threatening ID checks for the internet. No-one in a democracy should need to show their passport just to get online."
However, Chris Sherwood, chief executive of the NSPCC, said it strongly supported the move, adding, "Time is up for big tech".
"Now government must focus on holding them to account to ensure this transformational change for young people's safety is quickly delivered," he said.
Labour MP Jess Phillips resigned as safeguarding minister at the Home Office earlier this year, accusing Sir Keir of being slow to threaten legislation on preventing children taking naked images of themselves, despite the technology being available.
Additional reporting by Chris Vallance, Liv McMahon, and Laura Cress
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