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.
Home Secretary Priti Patel said she "fully supported" the move.
she said: "Tech companies like Facebook have a vital responsibility to balance privacy with the safety of vulnerable children."
"I fully support the continued efforts of the NSPCC and children's charities around the world to engage Facebook on this issue."
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.
Those signing the letter say Facebook has failed to address concerns about child safety.
The missive 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 seems inclined to blindfold itself," the letter says.
"We request you to recognise and accept that an increased risk of child abuse being facilitated on or by Facebook is not a reasonable trade-off to make.
"Children should not be put in harm's way either as a result of commercial decisions or design choices."
'Safety first'
A spokesman for Facebook said protecting the well being of children on its platform was "critically important" to it.
"We have led the industry in safeguarding children from exploitation and we are bringing this same commitment and leadership to our work on encryption," he said.
"We are working closely with child-safety experts, including NCMEC [the US National Center for Missing and Exploited Children], law enforcement, governments and other technology companies, to help keep children safe online."
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