The Russian government has started legal proceedings to block the Telegram messaging app in the country.
The Roskomnadzor media regulator is seeking the block because the firm has refused to hand over encryption keys used to scramble messages.
Telegram, which is based in Dubai, was given a deadline of 4 April to hand over the keys.
The company has refused, saying the way the service is built means it has no access to them.
Russia’s main security agency, the FSB, wants the keys so it can read messages and prevent future terror attacks in the country.
In its court filing, Roskomnadzor said the legal action was related to the FSB request and Telegram’s non-compliance 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”.
In a statement, he said: “The FSB’s requirements to provide access to private conversations of users are unconstitutional, baseless, which cannot be fulfilled technically and legally.”