The charity for child protection has appeared to Facebook head Mark Zuckerberg for their plans to unite messaging services, saying that it would help grooms make their way to children.
The idea of integration – including Instagram, WhatsApp, and Facebook – aims to facilitate conversations with friends.
For example, someone using Facebook can send a message to a friend using only WhatsApp.
But NSPCC says it will also make it easier for pedophiles to target more children.
Her online head safety Andy Burroughs said: "Bringing these messaging services together creates a larger pool for kids.
"We know grooms are sending messages on a scale – like phishing emails from crooks who are trying to steal money.
"Merging messages can make it easier."
As part of the change, all messages will be "end-to-end encrypted" – protection, which means that only the sender and the recipient can see them.
But Mr Burrows added: "Encryption makes it harder for social networks or law enforcement to see if grooms work."
Facebook owns all three separate services and seeks to unite messages by the end of this or 2020 year.
Networks will remain independent apps.
Social media consultant Matt Navarra said the Facebook move is partly in support of Apple's popular iMessage service.
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A Facebook spokesperson said: "We have lots of discussions and debates as we begin the long process of understanding the details of how it works."
The latest data for the whole year – 2017-18 – show that the police have registered more than 3,000 crimes, about nine per day.
Where the cops note the method used to communicate with a child, more than half of the cases (53%) occur on Facebook or the apps they own.