Facebook risks becoming a "one-stop grooming shop" if it presses ahead with plans to encrypt across all its messaging services, the NSPCC has warned.
The social networking site is considering end-to-end encryption on Messenger and Instagram Direct – on top of WhatsApp, which is already encrypted.
But there has been concern the move could prevent child abusers being caught.
Out of 9,259 instances where police in England and Wales said they know the platform used in child abuse image and online child sexual offences, just over 4,000 were carried out on Facebook, Instagram or WhatsApp.
The data – obtained by the charity from freedom of information requests to police forces between April 2018 and 2019 – shows that 22% were reported on Instagram, followed by 19% on Facebook or Facebook Messenger.
Only 3% (299 instances) were from WhatsApp, which the NSPCC says highlights how difficult it becomes to detect crimes on an end-to-end encrypted platform.
"Instead of working to protect children and make the online world they live in safer, Facebook is actively choosing to give offenders a place to hide in the shadows and risks making itself a one-stop grooming shop," said Andy Burrows, NSPCC's head of child safety online policy.
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"For far too long, Facebook's mantra has been to move fast and break things but these figures provide a clear snapshot of the thousands of child sex crimes that could go undetected if they push ahead with their plans unchecked."
Rob Jones, director of threat leadership at the National Crime Agency (NCA), said: "Facebook's current end-to-end encryption proposals could place key information about what their users say and do out of the reach of law enforcement investigations."
The NSPCC fears the true scale could be much higher, as only 32 of the 43 police forces approached provided information.
It wants supporters to sign an open letter to Facebook, proposing measures such as no end-to-end encryption for messages going to or coming from children's accounts.
The charity also wants adults' accounts not to be encrypted until and unless Facebook has solutions to ensure child abuse can be detected and that children's safety will not be compromised.