Evidence suggests Russia is planning "the biggest war in Europe since 1945", Prime Minister Boris Johnson has said.
He told the BBC's Sophie Raworth in an interview: "All the signs are that the plan has already in some senses begun."
Intelligence suggests Russia intends to launch an invasion that will encircle Ukrainian capital Kyiv, Mr Johnson said.
"People need to understand the sheer cost in human life that could entail," he said.
The prime minister was speaking from Munich, where world leaders are meeting for an annual security conference.
The latest estimates by the US government suggests that between 169,000 and 190,000 Russian troops are stationed along Ukraine's border, both in Russia and neighbouring Belarus - but this figure also includes Russian-backed rebels in eastern Ukraine.
Mr Johnson warned that any conflict could be "bloody and protracted", saying Russian President Vladimir Putin was possibly "thinking illogically about this" and did not "see the disaster ahead".
"I think it's vital for us all now to get over what a catastrophe it would be for Russia," he added.
He indicated that the UK and US would bring further sanctions against Russia than have been suggested before, including stopping its companies "trading in pounds and dollars" - a move that he said would "hit very, very hard".
Mr Johnson added: "The lesson of [the Russian seizure of Crimea from Ukraine in] 2014 is that you can't just let Vladimir Putin get away with it."
An invasion of Ukraine would strengthen, rather than weaken, Nato, he predicted, saying Western countries could not allow opposition forces to come to "the false conclusion that might is right".
"If [Mr Putin] thinks he's going to get less Nato as a result of this, he's totally wrong," Mr Johnson said. "He's going to get more Nato."
By Peter Saull, political correspondent
While a full-blown Russian invasion of Ukraine is yet to materialise, Boris Johnson's latest comments ratchet up the war of words.
The prime minister raising the prospect of a conflict as devastating as World War II is designed to focus minds.
The ongoing tensions in eastern Europe have given Mr Johnson, who's faced a myriad of problems at home, a chance to show leadership on the world stage.
And on the sidelines of the Munich security conference, he seems to have made some progress on the question of sanctions.
Stopping Russian companies' ability to trade in pounds and dollars is a new proposal designed to do serious damage to the country's finances.
Ukraine is not a member of Nato or the European Union but has close relations with both.
Russia is insisting its neighbour should not be allowed to join Nato, which it sees as a threat to its security.
Western nations have accused Russia of trying to stage a fake crisis on Ukraine's border, warning in recent weeks Mr Putin's forces could be preparing to invade at any time.
But Russia has denied the claims, saying the massed troops are just conducting military exercises.
Asked whether a Russian invasion was still thought to be imminent, Mr Johnson said: "I'm afraid that that is what the evidence points to. There's no burnishing it."
The prime minister said US President Joe Biden had told Western leaders that intelligence suggested Russian forces were not just planning on entering Ukraine from the east, via Donbas, but down from Belarus and the area surrounding Kyiv.
"I'm afraid to say that the plan we are seeing is for something that could be really the biggest war in Europe since 1945, just in terms of sheer scale," he said.
People needed to not only consider the potential loss of life of Ukrainians, but also of "young Russians", he added.
Mr Johnson was speaking after meeting Ukrainian President Volodymyr Zelensky and telling world leaders in a speech that any invasion of Ukraine by Russia would "echo around the world".
During a phone call on Sunday, the prime minister and French President Emmanuel Macron agreed the next week would be "crucial for diplomacy".
A No 10 spokeswoman said the two leaders updated one another on their "respective diplomatic efforts", including Mr Macron's call with Mr Putin on Sunday.
"The prime minister noted that President Putin's commitments to President Macron were a welcome sign that he might still be willing to engage in finding a diplomatic solution," the spokeswoman added.
She said Mr Johnson stressed Ukraine's "voice must be central" in any talks, and that the two leaders "underscored the need for President Putin to step back from his current threats and withdraw troops from Ukraine's border".
Labour said it stood "four-square behind the UK government and Nato, with shadow health secretary Wes Streeting telling Sky News: "There should be no doubt on the resolve of the UK, on a cross-party basis, and the resolve of Nato across the alliance."
But he also urged the government to do more to prevent the City of London being a "centre" for "Russian kleptocrats and money laundering".
Writing in the Sunday Telegraph, Home Secretary Priti Patel warned the crisis could bring an increased risk of cyber attacks in Britain.
She said past Russian cyber activity had included interference with UK media, telecommunications, and energy infrastructure, and the government expected to see an escalation of similar operations in the coming months.
In the same paper, Lindy Cameron, head of the National Cyber Security Centre (NCSC), said that "attacks targeting Ukraine's digital infrastructure could be felt here in Britain".
She said that, in 2017, malware used by the Russian military to target infrastructure in Ukraine "spiralled recklessly out of control to affect many other countries, including the UK'.
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