"The West was too slow to act over Russian aggression in Ukraine, Boris Johnson has told Ukrainian MPs.
Addressing the Ukrainian parliament via video link, he said Ukraine's allies "cannot make the same mistake" as they did over the 2014 Crimea invasion.
He said the West had been "too slow to grasp what was really happening" and "failed" to collectively impose sanctions against Vladimir Putin.
Details of £300m of extra military aid were also announced by Mr Johnson.
The military aid to be sent to Ukraine in the coming weeks will include electronic warfare equipment, a counter-battery radar system, GPS jammers and night-vision devices.
Introduced to MPs in Kyiv by the Ukrainian parliament's speaker, Mr Johnson said Ukraine's allies must be "humble" as the nation was previously invaded in 2014 when Russia seized Crimea, and began the war in Donbas.
"We cannot make the same mistake again," he said.
The UK prime minister praised Ukraine's resistance for standing up to Russian forces.
He said: "You have exploded the myth of Putin's invincibility and you have written one of the most glorious chapters in military history and in the life of your country.
"The so-called irresistible force of Putin's war machine has broken on the immoveable object of Ukrainian patriotism and love of country."
He also echoed the words of Prime Minister Winston Churchill, just as Ukraine's President Volodomyr Zelensky did when he addressed MPs at Westminster in March.
Speaking to Ukrainian MPs in the chamber holding Ukrainian and Union flags, he said: "This is Ukraine's finest hour, that will be remembered and recounted for generations to come.
"Your children and grandchildren will say that Ukrainians taught the world that the brute force of an aggressor counts for nothing against the moral force of a people determined to be free."
Following the speech - the first address to the Ukrainian parliament by a world leader since the conflict began - President Zelensky described the UK and Ukraine as "brothers and sisters"."
http://www.bbc.co.uk/news/uk-61297478