This all started when KGB successor Federal Security Service (FSB) demanded access to Telegram's decryption keys a year ago.
Friday's ruling clears the way for Russian communications regulators to order the country's ISPs to block the Telegram protocol or Telegram servers on their networks.
The Federal Service for Supervision of Communications, Information Technology and Mass Media (Roskomnadzor) entered web.telegram.org and t.me, used by Telegram messenger, in the list of prohibited websites, as Meduza reported.
In March of this year, Russia's Supreme Court ruled that Telegram was required to provide the FSB with decryption keys that the security agency had requested in the wake of a 2017 terrorist attack in St. Petersburg.
After Friday's ruling, Alexander Zharov, the head of Roskomnadzor, said he would not say the "exact time when the block on Telegram will begin", adding that it would be "soon".
In early April, the Presidential Council for Human Rights called Roskomnadzor to refrain from blocking Telegram messenger in Russian Federation and asked FSB to find other ways of legal access to the messages of users endangering the national security.
The free application, which lets people exchange messages, photos and videos in groups of up to 5,000 people, has attracted more than 200 million users since its launch in 2013.
However, he said on his page in the VK social network that he could not guarantee users would retain access to Telegram without using virtual private networks (VPNs).
Telegram Chief Executive Pavel Durov has repeatedly condemned Russia's efforts to compromise the app's security and privacy features, and he said last month that the court's threats "won't bear fruit". Telegram's lawyer told the newspaper that the messaging service plans to do so. Providers will block the app starting from April 16.