More than 90% of websites found to contain child sexual abuse featured "self-generated" images extorted from victims as young as three, according to an internet watchdog.
The Internet Watch Foundation (IWF) warned of a "shocking" rise in the number of under-10s being coerced, blackmailed, tricked or groomed into performing sexually online.
Data released by the anti-abuse charity shows a record 275,655 websites were found to contain child sexual abuse in 2023 - an 8% rise from the previous year.
Of those, 254,070, or 92%, contained "self-generated" images or videos, with children under the age of 10 featuring on 107,615 of the sites, and youngsters aged between three and six found on 2,500 of them.
Susie Hargreaves, chief Executive of the IWF, said: "The imagery extorted or coerced from primary school-aged children is now finding its way onto the most extreme, dedicated child sexual abuse sites in shocking numbers.
"What starts in a child's bedroom, over a webcam, is shared, traded, and harvested by committed and determined sexual predators. The IWF is seeing the results in unprecedented numbers. These criminals are ruthless."
The IWF said it investigated a record 392,660 reports of suspected child abuse imagery last year - 5% more than in 2022.
Around one in five (54,250) of the websites found to contain child abuse included the most severe form, known as Category A.
The latest figures come after a landmark National Police Chiefs' Council report released last week highlighted the "worrying trend" of "sextortion", where children are blackmailed with the threat of compromising images being sent to family or released on social media unless money is paid.
Ishmael Duncan, 24, was jailed for 18 years in December after posing as a model agency scout to blackmail young girls into sending indecent photos and videos, using Snapchat to coerce and threaten them.
He pleaded guilty to 50 offences against 28 victims as young as nine from countries including the UK, Canada, Australia and the US - but investigators think he contacted nearly 10,000 children.
Although Meta is not mentioned in the report, the IWF is among the charities, law enforcement agencies and government ministers critical of the tech giant's decision to roll out end-to-end encryption to Facebook Messenger.
They say the move, which brought the chat app in line with services like Signal and its own WhatsApp, will make it easier for predators to exploit young victims and harder to detect their crimes.
Security Minister Tom Tugendhat said: "This alarming report clearly shows that online child sexual abuse is on the rise, and the victims are only getting younger. And yet, despite warnings from across government, charities, law enforcement and our international partners, Meta have taken the extraordinary decision to turn their backs on these victims, and provide a 'safe space' for heinous predators.
"The decision to roll out end-to-end encryption on Facebook Messenger without the necessary safety features, will have a catastrophic impact on law enforcement's ability to bring perpetrators to justice.
"It isn't too late to work with us to keep children safe online. As Meta begins to implement default end-to-end encryption in the UK, they can and must ensure that robust safeguards are implemented at a time when children are at a greater risk online than ever before."
A Meta spokesperson said: "Encryption helps keep people, including children, safe from hackers, scammers and criminals.
"We don't think people want us reading their private messages so have spent years developing robust safety measures to prevent, detect and combat child abuse while maintaining online security.
"Our recently published report detailed these measures, such as restricting over-19s from messaging teens who don't follow them and using technology to identify and take action against malicious behaviour."