The channel was reportedly hacked with a fake text address broadcast which claimed to be the Ukrainian president calling on people to disarm. The cyberattack on Ukraine 24’s webstream is believed to have happened on Wednesday.
In the attack, Mr Zelensky reportedly said he was “capitulating” and would surrender.
The president swiftly released a video criticising the fake address, adding that it was not him.
He said he had never made any such statement and promised that the only people who should give up their arms are Russian soldiers.
Ukraine and Russia have both fallen victim to cyber-attacks during the conflict which is now in its third week.
Two days into the war, Ukraine’s Vice Prime Minister Mykhailo Fedorov said Kyiv would create an “IT army” to fight Moscow’s digital intrusions.
The official website of the Kremlin and the office of Russian President Vladimir Putin was taken offline last month in an apparent distributed denial of service (DDoS) attack.
Malicious, data-wiping software has also been discovered circulating in Ukraine, hitting hundreds of computers, according to researchers from the cybersecurity firm ESET.
Google said earlier this month that it has seen Russian hackers known to law enforcement engaging in espionage, phishing campaigns and other attacks targeting Ukraine and its allies in Europe.
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Mr Zelensky, in a video call to Mr Johnson and representatives from Baltic and northern European countries on Tuesday, expressed frustration Ukraine had not been allowed to join the military alliance.
But he said: “This is the truth and we have simply to accept it as it is.”
Mr Johnson, speaking to broadcasters at the Emirates Palace hotel in Abu Dhabi on Wednesday, said he understood what Ukraine’s leader was saying about NATO and the reality of the position.
The PM added: “And everybody has always said – and we’ve made it clear to Putin – that there is no way Ukraine is going to join NATO anytime soon.”
Mr Johnson said decisions about the future of the country had to be for the Ukrainian people and their elected leader, vowing Britain would continue to back Mr Zelensky.
Thousands of people have been killed in the conflict with millions more displaced.
Russia calls its actions a “special military operation” to “denazify” the country, a claim Ukraine and its allies have rejected as a pretext for an unjustified and illegal attack.