Boris Johnson warns that Russia is planning the biggest war in Europe since 1945.

Russia is planning “the biggest war in Europe since 1945,” Prime Minister Boris Johnson of Britain told the BBC on Sunday, warning that a conflict would be a “catastrophe” for Russia.

As the world nervously waits to see President Vladimir V. Putin’s intentions, Mr. Johnson said that all the signs showed that “the plan has already in some senses begun” and that the intelligence pointed to Russia’s intention to invade Ukraine.

“People need to understand the sheer cost in human life that could entail,” he told the BBC, noting that this applied to Russians as well as Ukrainians. He was speaking in Munich.

Mr. Johnson warned that a conflict in Ukraine would be a “catastrophe” for Russia, and suggested that Britain and the United States were prepared to ratchet up sanctions against Moscow beyond those that have already been outlined.

U.S. officials have vowed to unleash searing economic measures if Russia invades Ukraine, including sanctions on its largest banks and financial institutions, in ways that would inevitably affect daily life in Russia.

Mr. Putin has increasingly portrayed NATO’s eastward expansion as an existential threat to his country and insists that Moscow’s military buildup is a reaction to Ukraine’s deepening partnership with the alliance. Essentially, he appears intent on winding back the clock 30 years, to just before the collapse of the Soviet Union.

With NATO countries in Eastern Europe increasingly jittery about Russian aggression, the United States has announced plans to send troops nearly 3,000 supplementary troops to Poland and Romania.

Mr. Johnson warned that Mr. Putin’s actions would embolden rather than diminish NATO.

If Mr. Putin “thinks he’s going to get less NATO as a result of this,” he’s totally wrong,” Mr. Johnson was quoted as saying by the BBC. “He’s going to get more NATO.”