"I am afraid to say, but the tragedy we prophesied has happened, and it is worse than all our prophecies. We are witnessing the unfolding of the catastrophe on the European continent," CNN quoted Johnson.
"Obviously, Vladimir Putin is ready to use barbaric tactics against innocent civilians, bomb high-rise buildings, launch rockets into apartment buildings, kill children, and we are watching this more and more," he added.
Making a speech with his Polish counterpart in Warsaw, the British Prime Minister paid tribute to the "leadership and courage" shown by Ukrainian President Volodymyr Zelensky, who, he said, "inspired and mobilized" the world.
Johnson also noted that Russian President Putin underestimated "the passionate desire of the Ukrainian people to defend their country."
"I am absolutely convinced, and I am convinced more than ever: as this terrible conflict progresses, Putin will fail. I believe that Putin must fail and that we will be able to defend and preserve a sovereign, independent and democratic Ukraine," he added.
Earlier, the British Prime Minister called on the leaders of the G7 and other world leaders to continue to supply Ukraine with defensive weapons and put pressure on the Putin regime.