Russia is planning “the biggest war in Europe since 1945” in terms of scale, according to the Prime Minister.
Boris Johnson told the BBC that intelligence suggests Russia intends to launch an invasion that will encircle Ukrainian capital Kyiv.
He told the BBC’s Sophie Raworth in an interview: “All the signs are that the plan has already in some senses begun.”
He added: “People need to understand the sheer cost in human life that could entail.”
Mr Johnson was speaking from Munich, where world leaders are meeting for an annual security conference.
There are currently an estimated 150,000 Russian troops now posted around Ukraine’s borders.
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US president Joe Biden said on Friday he is now “convinced” Mr Putin has decided to invade Ukraine and assault the capital, Kyiv.
He reiterated that the assault could occur in the “coming days”.
Russia denies the claims and says troops are conducting military exercises in the region.
Mr Johnson was asked by the BBC whether a Russian invasion is still thought to be imminent.
He said: “I’m afraid that that is what the evidence points to, there’s no burnishing it.
“The fact is that all the signs are that the plan has already in some senses begun.”
Mr Johnson was speaking after meeting with Western leaders and the Ukrainian President Volodymyr Zelensky.
The full interview will be broadcast on BBC One’s Sunday Morning programme at 9am tomorrow (February 19).