A court in Moscow has approved a request from the Russian media regulator to block the Telegram messaging app immediately.
Security officials say they need to monitor potential terrorists.
But the company said the way the service was built meant it had no access to customers' encryption keys.
Telegram had missed a deadline of April 4 to hand over the keys.
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A suicide bomber who killed 15 people on a subway train in St Petersburg last April used the app to communicate with accomplices, the FSB said last year.
105saw Russia take a step toward Chinese-style internet censorship
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