Boris Johnson has been criticised for comparing Ukraine's fight against invasion to the United Kingdom's exit from the European Union.
The British Prime Minister said on Saturday Russia's invasion of Ukraine was a "turning point for the world", arguing a victory for Russian President Vladimir Putin's forces would herald "a new age of intimidation".
Speaking to a Conservative Party conference in Blackpool, north-west England, Mr Johnson said Mr Putin was "terrified" the example of a free Ukraine would spark a pro-democracy revolution in Russia.
"That is why he is trying so brutally to snuff out the flame of freedom in Ukraine and that's why it is so vital that he fails," Mr Johnson said.
"A victorious Putin will not stop in Ukraine. And the end of freedom in Ukraine will mean the extinction of any hope of freedom in Georgia and then Moldova.
"It will mean the beginning of a new age of intimidation across Eastern Europe from the Baltic to the Black Sea."
Mr Johnson praised Ukrainians' defence of their country, and he said Britons and Ukrainians shared an "instinct" "to choose freedom".
He pointed to the 2016 referendum in which British voters decided to leave the EU "because they wanted to be free to do things differently and for this country to be able to run itself".
Mr Johnson's comments caused anger among politicians in both the UK and Europe.
Britain's Ed Davey, leader of the opposition Liberal Democrats, called Mr Johnson "a national embarrassment".
Donald Tusk, the former president of the European Council, called the comments offensive.
"Boris, your words offend Ukrainians, the British and common sense," he wrote on Twitter.
Gavin Barwell, who served as chief of staff to former Conservative British prime minister Theresa May, said: "Voting in a free and fair referendum isn't in any way comparable with risking your life to defend your country against invasion."
Lord Barwell also pointed out that Ukraine had asked to join the EU.
Mr Johnson helped lead the campaign for the UK to leave the bloc it joined in 1973.
Britain's departure, eventually completed in 2020, remains highly divisive.
ABC/wires
