Priti Patel has warned that Facebook's intention to roll out encryption across its messaging services could threaten efforts to fight child abuse.
The UK's Home Secretary has cautioned against the encryption plan, which means that only the sender and recipient can read messages.
Patel said: "Sadly, at a time when we need to be taking more action, Facebook are pursuing end-to-end encryption plans that place the good work and progress achieved so far [on fighting the issue of child abuse] in jeopardy.
"The offending will continue, the images of children being abused will proliferate - but the company intends to blind itself to this problem through end-to-end encryption which prevents all access to messaging content."
She added: "This is not acceptable."
On the other hand, Jim Killock, executive director of the Open Rights Group, observed that encryption remains a complex issue.
He told the BBC: "Encryption is popular and growing because users want security and protection from fraud, scams and abuse of their data.
"It would be completely unreasonable to ban or limit everyday security for one set of people over the other.
"It is absolutely necessary to deal with child abuse images, but solutions must be compatible with people's right to keep themselves safe from other kinds of criminality, so should move away from endlessly debating how to stop the use of encryption."
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