Child-protection organisations say Facebook’s decision to strongly encrypt messages will give offenders a place to hide.
The company is moving ahead with plans to implement the measure on Facebook Messenger and Instagram Direct.
But more than 100 organisations, led by the NSPCC, have signed an open letter warning the plans will undermine efforts to catch abusers.
They say Facebook has failed to address concerns about child safety.
End-to-end encryption, already used on Facebook-owned WhatsApp, means no-one, including the company that owns the platform, can see the content of sent messages.
And the letter urges the company to stop the rollout of its plans until “sufficient safeguards” are in place.
“At a time when we could be looking to build upon years of sophisticated initiatives, Facebook instead