The Prime Minister of the United Kingdom first used the word "genocide" in his reference to revelations about the massacre of Ukrainian civilians in the city of Bukha, northwest of Kiev.
"I am afraid that when you look at what is happening in Bukhara, the revelations we see of what Putin has done in Ukraine, it does not seem to me that he is far from the genocide," Johnson said.
He added that the reaction of other countries is not surprising. "I have no doubt that the international community, with Britain at the forefront, will move again in a coordinated manner to impose more sanctions and more penalties on the Vladimir Putin regime," he said.
The British government had so far referred to an obvious war crimes committed by the Russian invaders, but had avoided the word "genocide". On Monday, Foreign Minister Liz Tras said it was up to the international courts to rule on such a matter.
Boris Johnson made the remarks during a hospital visit.
Earlier, British Health Minister Sajid Javid said the international community must act to stop the worst mass killings of civilians in Europe since the 1995 Srebrenica massacre.