Ukraine's national telecoms operator Ukrtelecom was hit by a 15-hour outage, where connectivity collapsed to 13 percent of pre-war levels.
The government said that the outage was due to a major cyberattack that has now been successfully defended against.
The outage was "the most severe registered since the invasion by Russia," Internet outage monitor NetBlocks said. The group called it an "ongoing and intensifying nation-scale disruption to service."
The State Service of Special Communications and Information Protection of Ukraine (SSSCIP) said: "Today, the enemy launched a powerful cyberattack against Ukrtelecom’s IT-infrastructure... In order to preserve its network infrastructure and to continue providing services to Ukraine’s Armed Forces and other military formations as well as to the customers, Ukrtelecom has temporarily limited providing its services to the majority of private users and business-clients.
"The specialists from the SSSCIP promptly reacted to the situation, due to which the attack was repelled. And now Ukrtelecom has an ability to begin restoring its services to the clients."
Cybersecurity firm Sophos said that the outage was linked to a DDoS attack that "appeared to be utilizing vulnerable WordPress sites, triggering amplification attacks every time someone visited a booby-trapped page."
Ukraine's monopolist telephone company, Ukrtelecom is the country's biggest provider of fixed Internet in terms of geographical coverage, but comes after Kyivstar in terms of number of clients.
The company's services were disrupted by combat damage in early March, and went down for 40m a week later due to a cyber attack.
Russia's unprovoked invasion has also damaged around 10 percent of Turkcell's infrastructure in Ukraine, and impacted telecoms infrastructure in Severodonetsk, Sumy Oblast, and Kyiv. ISPs Uacity, Volia, and Skyline have all seen services degrade as a result of the war.