Russia has been accused of "desperation" and using "dirty tricks" after senior government ministers received hoax calls from an impostor posing as Ukraine's prime minister.
A cross-Whitehall security inquiry has been launched after Home Secretary Priti Patel and Defence Secretary Ben Wallace were targeted.
Mr Wallace became suspicious that something was wrong during a 10-minute video call on Microsoft Teams on Thursday.
He claimed the caller asked about Britain's military movements - and has now publicly accused Russia of being behind it.
He ordered an investigation into the security breach and there are serious questions about Whitehall security after Patel said the same thing happened to her earlier this week.
Wallace said he terminated the call after "several misleading questions".
Wallace is understood to have been put through to the video call which lasted around 10 minutes by officials, rather than being dialled directly on his phone.
Wallace said it was a "desperate attempt" but "no amount of Russian disinformation, distortion and dirty tricks" could distract from the human rights abuses carried out during the invasion of Ukraine by Vladimir Putin's forces.
Armed Forces Minister James Heappey said: “He was acutely aware that he was on a Microsoft Teams call, and therefore he would not ever disclose any sensitive details on a platform that can be very easily intercepted and listened into.
"And the call therefore was pretty bland, I think, from what Ben was explaining to me last night, when the call started to ask more pointed questions about our intentions militarily in the region, Ben knew full well that’s not the sort of question anybody would normally ask on Teams.
"And so he became pretty suspicious and terminated the call.”
Home Secretary Ms Patel said she had also been targeted. "This also happened to me earlier this week," she tweeted. "Pathetic attempt at such difficult times to divide us. We stand with Ukraine."
Wallace went public because of fears that Russia may try to distort any footage or audio material from the call.
He said: "Today an attempt was made by an impostor claiming to be Ukrainian PM to speak with me.
"He posed several misleading questions and after becoming suspicious I terminated the call.
"No amount of Russian disinformation, distortion and dirty tricks can distract from Russia's human rights abuses and illegal invasion of Ukraine.
"A desperate attempt."
The video call was set up after an email, purportedly from an aide at the Ukrainian embassy, was sent to a government department and then forwarded to the Ministry of Defence.
The call was set up and Wallace was put through on Teams to the "prime minister of Ukraine", posing with the country's flag behind him.
After initial introductions and thanks for the UK's support, Wallace is understood to have became suspicious as the man started asking questions about British policy and eventually urged the defence secretary to shout slogans.
The level of sophistication involved in the hoax has convinced government sources that it was a Russian plot.
Senior ministry of defence sources fear Moscow may attempt to splice together Wallace's comments in an attempt to embarrass him.
