Telegram, founded by Russian-born entrepeneur Pavel Durov, is the most popular messaging service in Russia, boasting more than one billion monthly active users worldwide.
Russia's internet watchdog announced it was throttling access to the app on Tuesday, accusing it of failing to follow legislation.
It was not immediately clear if Russia planned to ban the application outright.
Russian officials -- including the Kremlin -- still use it to issue statements and it is the most popular outlet for pro-government military bloggers commenting on the Ukraine war.
Some have warned that blocking the app would hobble communications around the front line and in Russian-occupied territory.
Durov, who now lives outside Russia, has called the Kremlin's restrictions "an attempt to force its citizens to switch to a state-controlled app built for surveillance and political censorship".
He has clashed with the Russian authorities before.
He was forced out of the VK social media site he founded -- a Russian equivalent of Facebook -- under pressure from the authorities.
Russia spent two years trying to block Telegram, but having failed either to restrict access or to stop the growth of the service, it lifted the ban in 2020.