Mr Johnson said on Monday that “gloomy” intelligence suggested Russia was planning a lightning raid on Kyiv, prompting the order for British embassy staff and their families to begin leaving the Ukrainian capital.
The Prime Minister said: “The intelligence is very clear that there are 60 Russian battle groups on the borders of Ukraine, the plan for a lightning war that could take out Kyiv is one that everybody can see.
“We need to make it very clear to the Kremlin, to Russia, that that would be a disastrous step.”
He warned that the people of Ukraine would resist any invasion and “from a Russian perspective, [it] is going to be a painful, violent and bloody business”, he said, adding: “I think it’s very important that people in Russia understand that this could be a new Chechnya.”
Reflecting on Mr Biden’s misspoken suggestion that a “minor incursion” may result in a more measured response by the US and its allies, Mr Johnson said: “Any incursion, of any kind, of any dimension, into Ukraine is not going to be a cost-free business,” adding: “There will be casualties.”