The messaging app Snapchat has introduced end-to-end encryption, preventing the photos shared between its users being intercepted.
The app, which has 186 million users and an estimated 16 million in the UK, is encrypting billions of messages a day.
End-to-end encryption scrambles a message as it travels over an internet network so it can be read only by the recipient. Messages that are not encrypted can potentially be picked up by security services or hackers.
Other messaging apps including WhatsApp and iMessage use encryption, claiming the technology keeps users secure.
However, politicians and security chiefs have said it risks allowing criminals to communicate safely.
Snapchat security engineer Subhash Sankuratripati, speaking at a conference in California, said that the app had implemented the feature a year ago.
The encryption applies to "snaps" - picture and video messages sent between users, which disappear after being watched.
Other forms of message on Snapchat, such as text messages and group chats, are not end-to-end encrypted.
In the past, the company has encrypted messages, but the company held the keys that would have allowed them to be decrypted.
Although snaps disappear after being opened and were wiped from the company's servers, if users were unable to receive messages, they would be stored on the company's servers for up to 30 days.
The human rights group Amnesty International has said Snapchat's was "not protecting users' privacy".
Sankuratripati said the feature would provide "increased assurance around privacy to our users".
He added that the company planned to encrypt text and group chats in a similar way in the future.