Secure chat app Telegram along with a newer version of the software called Telegram X were removed from Apple's iOS App store Wednesday (31 January), with little explanation.
A thread surfaced on Reddit that highlighted the disappearance of the app, which UK prime minister Theresa May recently claimed was a safe haven for "criminals and terrorists". Telegram's founder, Pavel Durov, said on Twitter that it should be back online soon.
Durov, who launched the app in 2013, tweeted on Thursday: "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."
A search on the marketplace for Telegram resulted in no matches. It brought up Viber, WeChat, Facebook Messenger and Google Hangouts.
It appears content guidelines were breached but remains unclear what "inappropriate content" was shared. Telegram has, in the past, been linked to the spread of extremist and terrorist material.
It is encrypted end-to-end and was allegedly once the app of choice for the Islamic State.
It remains popular with citizens living under heavy state censorship. Alongside Instagram, it was blocked by the Iranian government during the anti-state protests in late December.
The mysterious removal came as the company launched Telegram X for Android, an update that promises to be "faster, slicker and easier to use". A test version was listed for iOS.
Its strong stance on user protection has angered politicians and law enforcement. During a speech at the World Economic Forum in Switzerland on 25 January, May urged technology companies to "do more" to stop illegal activity from taking place on their platforms.
She said: "[We] need cross-industry responses because smaller platforms can quickly become home to criminals and terrorists. We have seen that happen with Telegram.
"And we need to see more co-operation from smaller platforms like this. No-one wants to be known as 'the terrorists' platform' or the first-choice app for paedophiles."
Last month, security experts from Symantec said that hackers had uploaded a fake version of Telegram to the Google Play Store. Upon analysis, it was a strain of adware – software that gains illicit revenue through the process of displaying fraudulent advertisements.