Boris Johnson will address the Ukrainian parliament on Tuesday, becoming the first world leader to do so since the outbreak of war in February.
In a recorded address, the prime minister will praise Ukraine's resistance against tyranny and announce a new £300m package of military aid to support the country's ongoing defence against Russia.
In his address to the Verkhovna Rada on Tuesday, the prime minister is also expected to hail Ukraine's "finest hour" and to stress that the UK is "proud to be among their friends".
His comments will be broadcast as the British embassy reopens in Kyiv in a show of support for Ukraine.
Mr Johnson is expected to say: "When my country faced the threat of invasion during the Second World War, our parliament - like yours - continued to meet throughout the conflict, and the British people showed such unity and resolve that we remember our time of greatest peril as our finest hour."
He will add: "This is Ukraine's finest hour, an epic chapter in your national story 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."
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The £300m military aid package will include electronic warfare equipment, a counter battery radar system, GPS jamming equipment and thousands of night vision devices.
The UK will also send more than a dozen new specialised Toyota Landcruisers in the coming weeks to help protect civilian officials in eastern Ukraine and evacuate civilians from frontline areas.
This follows a request from the Ukrainian government.
Volodymyr Zelenskyy, the Ukrainian president, addressed British MPs via video link in March on the 13th day of the war.
He asked the UK for more arms and repeated calls for a no-fly zone over Ukraine.
"We will not give up and we will not lose," President Zelenskyy told MPs in the historic address that echoed Winston Churchill's famed wartime address.
"We will fight until the end, at sea, in the air. We will continue fighting for our land, whatever the cost.
"We will fight in the forests, in the fields, on the shores, in the streets."
Mr Johnson's address follows his surprise visit to Kyiv to meet Mr Zelenskyy last month, when he praised his "resolute leadership and the invincible heroism".