Western officials have warned in recent weeks that Russia could be preparing to invade at any time, but Russia has denied the claims, saying troops are conducting military exercises in the region.
Asked whether a Russian invasion is 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 fact is that all the signs are that the plan has already in some senses begun."
The prime minister said US President Joe Biden had told Western leaders 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," the prime minister said.
