UK Prime Minister Boris Johnson told Ukrainian MPs on Tuesday that the country's defence against the Russian invasion was its "finest hour".
Speaking over a video link from Downing Street, he announced a new package of aid for Ukraine, containing military and civilian equipment.
Johnson delivered the speech to Ukraine's parliament on the 69th day of the conflict, referencing the UK's World War II-era leader Winston Churchill.
"I have one message for you today, Ukraine will win," he said, describing Ukraine's resistance as "one of the most glorious chapters in military history".
"Ukrainians taught the world that the brutal force of an aggressor counts for nothing against the moral force of people determined to be free," Johnson added.
He is the first world leader to address the Verkhovna Rada since Russia invaded on 24 February.
In the address, the UK PM praised Ukrainian resistance for "beating back" Russian armed forces, saying "it is not the size of the dog in the fight, it is the size of the fight in the dog".
"Your farmers kidnapped Russian tanks with tractors, pensioners stood up in the road and told Russian soldiers to hop it, although they may have used more colourful language than that," Johnson said.
He continued: "Even in areas that were temporarily captured, indomitable populations turned out to protest day after day."
Johnson condemned Putin's invasion of Ukraine as "grotesque" and "illegal", pointing to the alleged war crimes Russian soldiers have committed across Ukraine.
One reason for the war, Johnson believed, was authoritarianism and the lack of democratic freedoms in Russia.
"When there are no limits on Putin's power, that is where he makes catastrophic mistakes," he said. "What he has done [in Ukraine] is an advertisement for democracy."
Citing Russia's seizure of Crimea in 2014, he said "we were too slow to grasp what was happening" and that the west had "collectively failed" to impose sanctions.
"We cannot make the same mistake again."
The British PM then detailed further military aid to Ukraine worth £300 million (€358 million), saying the UK was "proud" to be the country's friend.
Electronic warfare equipment, radar systems to detect Russian artillery, GPS jamming equipment and thousands of night vision devices were listed by Johnson, alongside specialised vehicles to help evacuate Ukrainian civilians.
He echoed an announcement by British ministers last week that the UK would send Brimstone anti-ship missiles and STORMER air defence vehicles to Ukraine.
These have been critical in slowing Russia's advance.
Johnson said the UK would carry on supplying Ukraine "until no one will ever dare attack you again," adding "no outsider can speak lightly about how this conflict can be settled."
He called the conflict a moral battle of "good vs evil".
This "conflict has no moral ambiguities", said Johnson. Ukrainians have a right to protect themselves against "Putin's deranged imperialism".
He ended the speech by saying "good luck, we are with you."
His visit to Ukraine comes amid mounting tensions between the UK and Russia.
In April, UK Foreign Secretary Liz Truss provoked Russian ire when she called for its forces to be "pushed out" of Ukraine and asked the west to "double down on arms supplies."
This came off the back of remarks by another UK minister that it would be acceptable for Ukraine to strike targets in Russia with British weapons.
The Kremlin responded that such actions threatened European security and could potentially lead to World War Three.