The NSPCC has warned Facebook that it risks becoming a "one-stop grooming shop" if it presses ahead with plans to encrypt across all its messaging services.
Facebook is considering end-to-end encryption on Facebook Messenger and Instagram Direct - on top of WhatsApp, which is already encrypted - but there has been a deep concern that 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 per cent were reported on Instagram, followed by 19 per cent on Facebook or Facebook Messenger.
Only 3 per cent (299 instances) were from WhatsApp, which the NSPCC says highlights how difficult it becomes to detect crimes on an end-to-end encrypted platform.
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The charity believes criminals will be able to carry out more serious child abuse on Facebook's apps undetected without needing to lure them off to encrypted platforms, if it goes ahead with changes.
"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.
"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.
"If Facebook fails to guarantee encryption won't be detrimental to children's safety, the next Government must make clear they will face tough consequences from day one for breaching their duty of care."
The NSPCC fears the true scale could be much higher, as only 32 of the 43 police forces approached provided information.
It is calling for 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.
Heightened concerns come after the Home Secretary Priti Patel and her counterparts in the US and Australia wrote to Facebook chief Mark Zuckerberg, explaining how such a move could hinder law enforcement trying to investigate child abusers and terrorists operating online.