Russia risks creating 'new Chechnya' if it invades Ukraine, says Boris Johnson
Johnson said he did not think an invasion was inevitable, despite what he described as "gloomy" intelligence about the build-up of Russian forces on the border with Ukraine.
But he told reporters: "Invading Ukraine, from a Russian perspective, is going to be a painful, violent and bloody business.
"I think it's very important that the people in Russia understand this could be a new Chechnya."
Johnson's comments echo that of his Foreign Secretary Liz Truss in Sydney last week, where she said President Vladimir Putin had "not learned the lessons of history".
"Invasion will only lead to a terrible quagmire and loss of life, as we know from the Soviet-Afghan war and conflict in Chechnya," she said.
Truss' department on Saturday alleged that it had information the Kremlin was seeking to install a pro-Russian leader in Kyiv, in an unusual diplomatic step.
It did not release details of the evidence, which a US official called "deeply concerning". Russia dismissed the claims as "disinformation".
Britain is among a handful of Western nations that has sent weapons to Ukraine and is pushing for tough economic sanctions in the event of any Russian incursion.
Johnson said Britain stood "four square" with Ukraine and supported its territorial sovereignty, warning Russian aggression would not be "cost-free".
The Prime Minister's official spokesman said separately: "There are absolutely no plans to send UK combat troops to Ukraine."