Boris Johnson has warned Vladimir Putin is "sowing the seeds of catastrophe" for himself and declared " Ukraine will win" in an address to its Parliament.
Speaking to the Verkhovna Rada by video link, the Prime Minister hailed the courage of people of Ukraine and said they would triumph over Russia's "grotesque and illegal campaign".
Ukrainian President Volodymyr Zelensky watched on as Mr Johnson hailed the resistance against Putin's forces as Ukraine's "finest hour", in an echo of Winston Churchill's Commons speech on June 18 1940 as he rallied the country for the Battle of Britain.
Mr Johnson used his address - the first by a world leader - to confirm £300 million in military support for Ukraine and to warn that Russia's brutal assault would fail.
He told Ukrainian MPs: "Day after day, missiles and bombs continue to rain on the innocent people of Ukraine.
"In the south and the east of your wonderful country, Putin continues with his grotesque and illegal campaign to take and hold Ukrainian soil, and his soldiers no longer have the excuse of not knowing what they are doing.
"They are committing war crimes and their atrocities emerge wherever they are forced to retreat - as we've seen at Bucha, at Irpin, at Hostomel and many other places.
"We in the UK will do whatever we can to hold them to account for these war crimes and in this moment of uncertainty, of continuing fear and doubt, I have one message for you today: Ukraine will win, Ukraine will be free."
The PM said the Ukrainians had "exploded the myth of Putin's invincibility" with their courage, adding: "This is Ukraine's finest hour, that will be remembered and recounted for generations to come."
Mr Johnson warned that Putin had "sowed the seeds of catastrophe" for himself and for his country by invading Ukraine.
He told MPs: "As you turned the Russian army back from the gates of Kyiv, you not only accomplished the greatest feat of arms of the 21st century, you achieved something deeper and perhaps equally significant.
"You exposed Putin's historic folly, the gigantic error that only an autocrat can make."
Accusing the Russian President of ruling by fear and gagging his critics, Mr Johnson added: "When an autocrat deliberately destroys these institutions, he might look as though he is strong and some people might even believe it, but he is sowing the seeds of catastrophe, for himself and for his country, because there will be nothing to prevent him committing another terrible mistake."
The PM admitted Western allies had made a mistake in failing to confront Putin over the annexation of Crimea in 2014.
"We cannot make the same mistake again," he said.
President Zelensky spoke after Mr Johnson, describing the UK and Ukraine as now "brothers and sisters".
It comes nearly two months after Mr Zelensky addressed the House of Commons over video link.
Mr Zelensky also gave a nod to Churchill in his speech, where he vowed to fight Putin's forces "in the forests, in the fields, on the shores, in the streets."
The Prime Minister met President Zelensky on a surprise visit to Kyiv last month.
Meanwhile, the UK Ambassador to Ukraine, Melinda Simmons, has returned to Kyiv as the UK reopens its embassy.
