Ukrainian refugees rest at a hotel ballroom that has been converted into a makeshift shelter in the town of Suceava, Romania on March 15, 2022. AFP
"As time goes on, and as the number of Russian atrocities mounts up, I think it becomes steadily more difficult and politically embarrassing for people either actively or passively to condone Putin's invasion," he told the Sunday Times.
There were now "considerable dilemmas" for countries who were yet to speak out against Russian President Vladimir Putin, he said, adding: "I think that in Beijing you are starting to see some second thoughts."
Ukraine on Saturday called on China to condemn "Russian barbarism" after dozens were killed in new strikes.
Ukraine and the United States are concerned about Chinese potentially sending military aid to Russia or helping Moscow circumvent Western sanctions.
US President Joe Biden has warned Chinese leader Xi Jinping of "consequences" if he backs Russia, but Beijing has so far showed no sign of criticising the invasion.
Johnson called the crisis a "turning point for the world" during a speech at his Conservative Party conference attended by the Ukrainian ambassador to London, Vadym Prystaiko.
"There are some around the world... who say that we're better off making accommodations with tyranny... I believe they are profoundly wrong," the British leader told the conference in Blackpool, northwest England.
"To try to renormalise relations with Putin after this, as we did in 2014, would be to make exactly the same mistake again, and that is why Putin must fail.
"This is a turning point for the world and it's a moment of choice. It's a choice between freedom and oppression," Johnson said.