Britain has pledged Stg 300 million ($A531 million) more in military aid to Ukraine, including electronic warfare equipment and a counter-battery radar system, on top of around Stg 200 million of assistance so far.
The UK has sent Ukraine more than 5000 anti-tank missiles and five air defence systems as well as other munitions and explosives since Russia's invasion on February 24, which has destroyed cities and left thousands of people dead or injured.
The United States has provided $US3 billion ($A4.3 billion) of military aid to Ukraine so far, and last week President Joe Biden asked Congress to approve more than $US20 billion more in military support.
Russia last week said NATO countries were in effect engaging in a proxy war by providing arms to Ukraine, where it says it is conducting a "special operation" to disarm the country and rid it of anti-Russian nationalism fomented by the West.
Ukraine and the West say Russia launched an unprovoked war of aggression to seize territory, which threatens to spiral into a much wider conflict.
Prime Minister Boris Johnson's office said that as well as the other equipment, Britain would offer night-vision devices, tools to jam satellite navigation, heavy-lift drones to resupply Ukrainian troops, and armoured cars for civilian officials.
Johnson intends to address Ukraine's parliament on Tuesday morning via videolink.
"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," Johnson plans to say, according to a speech extract provided by his office.
While Britain has provided significant military aid, it has so far accepted relatively few of the more than five million Ukrainians who have fled their country.
As of last week, Britain's government said it had issued 86,100 visas to Ukrainians, of whom 27,100 had reached Britain.