Boris Johnson will quote his hero Winston Churchill as he pays tribute to Ukraine 's resistance against Russia.
The Prime Minister addresses the Ukrainian parliament via video link - becoming the first world leader to deliver a speech to the Verkhovna Rada since Vladimir Putin 's troops invaded on February 24.
The PM, who has written a biography of Britain’s wartime leader, will repeat one of his most famous lines.
On June 18, 1940 - rallying the country for the looming Battle of Britain that summer - Churchill told the Commons: “Let us therefore brace ourselves to our duties, and so bear ourselves that, if the British Empire and its Commonwealth last for a thousand years, men will still say, ‘This was their finest hour’.”
Speaking from the Downing Street press briefing room, Mr Johnson will tell Ukrainian parliamentarians: “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.
“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.”
Mr Johnson’s speech comes nearly two months after Ukrainian President Volodymyr Zelensky addressed the Commons over video link.
The Ukrainian leader, who has been widely praised for his communication skills, echoed Churchill in his own address to Parliament 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.
As the British embassy reopens in the capital Kyiv, the PM will say the UK is “proud to be among Ukraine’s friends”.
He is expected to announce a £300million package of “defensive military aid for Ukraine”.
The vital kit will include electronic warfare equipment, a “counter battery radar system”, GPS jamming equipment and thousands of night vision devices.
Britain will also send drones which can airlift gear to Ukrainian forces in the field, as well as more than a dozen new Toyota Land Cruisers to help protect officials in eastern Ukraine and evacuate civilians from the frontline.
