A Russian court in Moscow has approved a request from the media regulator in Russia to block the Telegram messenger immediately.
The media regulator sought to block the messenger because the company had refused to hand over encryption keys used to scramble messages. BBC News reported.
FSB officials say they need to monitor potential terrorists.
But the company said the way the service was built meant it had no access to customer’s encryption keys.
The telegram messenger had missed a deadline of 4 April 2018 to hand over the keys.
The FSB security agency has said Telegram is the messenger of choice for “international terrorist organizations in Russian Federation”.
A suicide bomber who killed 15 people on a subway train in St. Petersburg last year in April used Telegram to communicate with accomplices; the security agency said last year.
The Telegram messenger is also widely used by the Russian authorities.
In its court filing, Russian media regulator Roskomnadzor said Telegram messenger had failed to comply with its legal requirements as a “distributor of information”.
Telegram’s attorney, Pavel Chikov, said the official attempt to stop the messenger being used in Russia was “groundless”.
“The FSB’s requirements to provide access to private conversations of specific users are baseless and unconstitutional, which cannot be fulfilled technically” Chikov added.
The app is widely used in Russia and many nations in the Middle East, as well as around the rest of the world. It says it has more than 200 million users.
Its popularity has grown because of its emphasis on encryption, which thwarts many widely used methods of reading confidential communications.
It allows groups of up to 5,000 people to send messages, pictures, and videos without charge and with complete encryption.
The Telegram messenger has been used by the Islamic State group and its supporters though the firm says it has made efforts to close down pro-IS channels.