All Facebook and Messenger chats will be encrypted automatically, parent company Meta has announced, El.kz cites BBC.
Messages and calls protected by end-to-end-encryption (E2EE) can be read only by the sender and recipient.
It has been possible to opt in to encrypted messages for years, but now it will become the default position.
Critics, including the UK government and police, claim the move to default encryption will make it harder to detect child sexual abuse on Messenger.
The Home Secretary, James Cleverly, said he was "incredibly disappointed" by Meta's decision after working together to tackle other online harms.
"We'll continue to work closely with them (Meta) to keep children safe online, but we must be honest that in our view, this is a significant step back", he said.
James Babbage, director general for threats at the National Crime Agency, was also highly critical.
"It is hugely disappointing that Meta is choosing to roll out end-to-end encryption on Facebook Messenger.
"Today our role in protecting children from sexual abuse and exploitation just got harder," he said.