The UK will provide an extra £1bn of military aid for Ukraine, offering equipment enabling the country to go on the front foot to regain lost territory.
The new funding amounts to a 77% increase on the £1.3bn already provided, with Boris Johnson claiming British support was "transforming Ukraine's defences" against the Russian onslaught.
At the NATO summit in Madrid, ending 30 June, the UK promised funding towards capabilities including sophisticated air defence systems, drones and electronic warfare equipment.
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British officials said the equipment represents the first step in enabling Ukraine to go beyond defence and carry out offensive operations against Russian ground forces in order to recover lost territory.
Mr Johnson said: "Putin's brutality continues to take Ukrainian lives and threaten peace and security across Europe.
"As Putin fails to make the gains he had anticipated and hoped for and the futility of this war becomes clear to all, his attacks against the Ukrainian people are increasingly barbaric.
"UK weapons, equipment and training are transforming Ukraine's defences against this onslaught.
"And we will continue to stand squarely behind the Ukrainian people to ensure Putin fails in Ukraine."
The pledge for extra support comes after Ukrainian President Volodymyr Zelensky urged alliance leaders to do more to help his country resist Vladimir Putin’s invasion.
The UK is counting its support for Ukraine within its defence spending, taking its commitment to around 2.3% of gross domestic product.
Defence Secretary Ben Wallace said: "Britain's commitment to Ukraine is real and constant and we will stand by them until Russia changes course.
"This military assistance will help them intensify their fight against Russian aggression and ensure they have the defence capabilities they need."
On the 2.3% spending figure, Mr Wallace told reporters in Madrid: "To be fair No 10 does say 'if you include the extra Ukrainian spend', they have put a sort of caveat to that.
"Because, of course, it isn't core defence spending. I mean, it is not my core budget, it doesn't buy me any more planes, tanks or ships.
"It is obliquely helping Britain's defence because we’re helping Ukraine."