'UK Home Secretary Priti Patel has sent an open letter to Facebook calling on the firm to rethink its plans to encrypt all messages on its platforms.
The policy threatens "lives and the safety of our children", she said.
She feels it could hamper international efforts to grant law enforcers faster access to private messages on social media, as agreed between the UK and US.
Facebook said: "We believe people have the right to have a private conversation online."
The head of Facebook-owned WhatsApp Will Cathcart posted on Hacker News: "End-to-end encryption protects that right for over a billion people every day."
He added that he was not aware of "discussions that would force us to change our product".
Facebook said it is "consulting closely with child safety experts, governments and technology companies and devoting new teams and sophisticated technology" to keep people safe.
The agreement signed by Ms Patel - known as the Cloud Act - is designed to remove the barriers to cross-border surveillance.
It allows British law-enforcement agencies to demand from US tech firms data relating to terrorists, child-sexual abusers and other serious criminals.
It is hoped it will dramatically speed up investigations - previously, the process of requesting data from US firms could take anything from six months to two years.'
Read more: Facebook encryption threatens public safety, says minister
