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 4 April to hand over the keys. Russia's main security agency, the FSB, has said Telegram is the messenger of choice for "international terrorist organisations in Russia".
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.
The app is also widely used by the Russian authorities, Reuters news agency reports. In its court filing, media regulator Roskomnadzor said Telegram had failed to comply with its legal requirements as a "distributor of information".
Telegram's lawyer, Pavel Chikov, said the official attempt to stop the app being used in Russia was "groundless".
"The FSB's requirements to provide access to private conversations of users are unconstitutional, baseless, which cannot be fulfilled technically and legally," he said.