Boris Johnson has said Russia is planning "the biggest war in Europe since 1945" according to evidence seen by the Prime Minister.
Speaking to the BBC, the British PM added that "all the signs are that the plan has already in some senses begun."
The plan, the PM said, involves an invasion of neighbouring Ukraine that will encircle the country's capital of Kiev.
Mr Johnson was speaking from the annual Munich Security Summit, where he met with NATO allies who fear a Russian invasion is in motion, with estimates Vladimir Putin now has between 169,000 and 190,000 troops massed on Ukraine’s borders.
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But Mr Johnson and other world leaders still hope it may still be possible to avoid war.
Mr Johnson told the BBC : "People need to understand the sheer cost in human life that could entail.
"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."
He added: "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."
Russia plans on not only invading Ukraine from the east, via Donbas, but from Belarus in the north and the area surrounding the Ukrainian capital, the PM said, citing intelligence relayed to western leaders by US President Joe Biden.
After a day of shelling and false flag attacks in eastern Ukrainem western leaders warned the wheels of Russia’s war machine are already turning.
Mr Johnson today branded reports of Russian withdrawal “fake” and warned the “omens are grim”.
Mr Johnson told NATO allies at the summit: “At this moment of extreme danger for the world, it has seldom been more vital to preserve our unity and resolve.
“After a generation of freedom, we’re now staring at a generation of bloodshed and misery.”
Following crisis talks with Mr Biden and the leaders of France, Germany, Italy, NATO and the EU, Mr Johnson added: “We should not underestimate the gravity of this moment."
He pledged to ban Russians from raising cash on London’s money markets and to delve into the ownership of companies to sanction those really in control if Ukraine is invaded.
Mr Johnson also urged European countries to “wean” themselves off Russian President Putin’s oil and gas.
The UK is now sending aircraft carrier HMS Prince of Wales and 3 Commando Brigade to the region to reinforce our 2,000 troops in Estonia and 600 in Poland. A further 1,000 soldiers are on standby to give humanitarian aid.
Putin, who with Belarusian leader Alexander Lukashenko yesterday watched nuclear drills from his situation centre in Moscow, continues to deny he plans to invade Ukraine.
But President Biden cited US intelligence reports that an invasion will happen this week.
Mr Johnson did hold out an olive branch to Putin saying he was prepared to hold talks on the presence of NATO forces in countries neighbouring Russia.
He said: “NATO is a peaceful and defensive alliance and we are willing to work with President Putin to demonstrate that.”
- Mr Johnson will be on BBC One's Sunday Morning programme at 9am.
