Russia was behind three hoax calls to UK Cabinet ministers, Downing Street has officially confirmed.
A cross-Whitehall security inquiry has been launched after Home Secretary Priti Patel and Defence Secretary Ben Wallace were targeted in what Downing Street believes was a Russian plot.
A third attempt was made to contact Culture Secretary Nadine Dorries, which was unsuccessful.
On Monday, the Prime Minister's official spokesman said the hoax calls were "straight from the Kremlin playbook".
It is the first time Britain has officially blamed Russia for the disinformation attack.
The PM's spokesman said: "The Russian state was responsible for the hoax telephone calls made to UK ministers last week.
"This is standard practice for Russian information operations and disinformation is a tactic straight from the Kremlin playbook, to try and distract from their illegal activities in Ukraine and the human rights abuses being committed there.
"We are seeing a string of distraction stories and outright lies from the Kremlin, reflecting Putin's desperation as he seeks to hide to settle the conflict and Russia's feelings on the battlefield."
The alarm was raised by Mr Wallace after he became suspicious during a 10-minute video call on Microsoft Teams on Thursday, supposedly with the Ukrainian prime minister.
Downing Street has insisted Britain’s national security was not compromised, despite Ms Patel and Ms Dorries also being targeted.
Armed Forces Minister James Heappey said that Mr Wallace was “very cross” as it should not have been allowed to happen.
“Ben’s suspicions were aroused when somebody if he was who he said he was would have known full well that you don’t discuss military movements on Teams because the Russians are watching,” Mr Heappey told the BBC Radio 4 Today programme.
“Priti, like Ben, day in day out is dealing with matters of national security. When you deal with matters as sensitive as those great offices of state do, you get an instinctive feeling for what you should be saying on each means of communication.
“I have every confidence that Priti would have known she was on a Teams call. However long it went on, the content would have been appropriate to a Teams call.”
Mr Wallace said he became suspicious and terminated the call after “several misleading questions”.
It is understood he was put through to the video call which lasted around 10 minutes by officials, rather than being dialled directly on his phone.
Ms Patel later disclosed that she had also been targeted although the Home Office declined to give further details.
The PM's spokesman added: "We know this is something that the Russian state tries repeatedly.
"It won't be the last attempt to assure we do generally have strong protections in place to guard against this sort of thing and as we are doing today, we will call it out when we see it."
He went on: "As I say, there are myriad attempts by Russian state and others almost daily to attempt similar things, be it cyber hacking or other methods, and the vast, vast majority are stopped.
"And clearly, what happened here was not met supposed to happen. That's why there's an inquiry."
Mr Wallace went public because of fears that Russia may try to doctor footage or audio from the call for propaganda purposes.
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 Defence Secretary was put through to the “prime minister of Ukraine”, posing with the country’s flag behind him.
After initial introductions and thanks for the UK’s support, Mr 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.
