"We were alerted by Apple that inappropriate content was made available to our users and both apps were taken off the App Store. Once we have protections in place we expect the apps to be back on the App Store," he tweeted. Durov did not go into further detail about the inappropriate content or the protections needed.
Telegram X is a recently-released alternative Telegram client built in Swift, with higher-speed, slicker animations, themes, and more efficient battery use. The Android version of the Telegram app -- which has over 100 million downloads -- remained available to download from Google's Play app store, as did Telegram X.
In a second tweet at 4pm GMT, Durov said both Telegram and Telegram X have now reappeared in the Apple App Store.
"Telegram is back in the AppStore after being absent there since midnight CET. Every day 500,000+ users download Telegram for Android and another ~100,000 download Telegram for iOS," he said.
Telegram markets itself as more secure than mass-market messaging systems like WhatsApp -- for example, Telegram's secret chats use end-to-end encryption, leave no trace on the company's servers, support self-destructing messages, and don't allow forwarding. In addition, secret chats are not part of the Telegram cloud and can only be accessed on their devices of origin. Messages in Secret Chats use client-client encryption, while Cloud Chats use client-server/server-client encryption, and are stored encrypted in the Telegram Cloud.
However, the app has attracted criticism from politicians, most recently from UK prime minister Theresa May, for allowing criminals and terrorists to communicate. Telegram said it does block terrorist bots and channels (such as those related to Isis), but it will not block anybody who peacefully expresses "alternative opinions".
Apple had not responded to a request for comment at the time of publication.