The figures emerge as the UK is among seven nations warning of the impact of end-to-end encryption on public safety online.
Facebook was responsible for 94% of the 69 million child sex abuse images reported by US technology companies last year.
The figures emerged as seven countries, including the UK, published a statement on Sunday warning of the impact of end-to-end encryption on public safety online.
Facebook has previously announced plans to fully encrypt communications in its Messenger app, as well as its Instagram Direct service – on top of WhatsApp, which is already encrypted – meaning no one apart from the sender and recipient can read or modify messages.
The social media site said the changes are designed to improve user privacy on all of its platforms.
But law enforcement agencies fear the move will have a devastating impact on their ability to target paedophiles and protect children online.
Some 16.9 million referrals were made by US tech firms to the National Centre for Missing and Exploited Children (NCMEC) last year, including 69 million images of children being abused – up 50% on the previous year.
Some 94% of the reports, which include the worst category of images, came from Facebook, Home Office officials said.
But the National Crime Agency (NCA) has warned the number could drop to zero if Facebook presses ahead with end-to-end encryption.
Robert Jones, the NCA director responsible for tackling child sexual abuse, said of the plan: “The lights go out, the door gets slammed, and we lose all of that insight. It is as simple as that.
“And nothing, you know we’re relying on the best technical expertise… in the UK, the same people that keep the UK safe against terrorists, hostile states, cyber attacks, are telling us there is no viable alternative. I believe them. And I am deeply concerned.”
The NCA believes there are at least 300,000 people in the UK who pose a sexual threat to children, with 86,832 UK-related referrals to NCMEC last year, including 52% from Facebook and 11% from Instagram.
Mr Jones said industry reporting led to the arrest of more than 4,500 offenders and the safeguarding of around 6,000 children in the UK in the year to June 2020.