The UK is to send 120 armoured vehicles to Ukraine.
Downing Street confirmed the decision after Boris Johnson made a surprise visit to Kyiv on Saturday.
The Prime Minister said the West will supply Ukraine with the equipment it needs to ensure it can never again be invaded by Russia.
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Alongside armoured vehicles, Britain will send new anti-ship missile systems.
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Appearing with President Zelensky in a recorded broadcast, the Prime Minister said the West would continue to "ratchet up" sanctions on Moscow as he praised the courage of the Ukrainian resistance.
"I think that the Ukrainians have shown the courage of a lion, and you, Volodymyr, have given the roar of that lion," he said.
"The UK and others [will] supply the equipment, the technology, the know-how, the intelligence, so that Ukraine will never be invaded again.
"So Ukraine is so fortified and protected – that Ukraine can never be bullied again. Never be blackmailed again. Never be threatened in the same way again."
The latest support package represents a significant stepping up of UK military assistance to Ukraine.
It came just a day after the PM announced a further £100m worth of kit for Kyiv, including anti-tank and anti-aircraft weaponry and so-called "suicide drones", which loiter over the battlefield before attacking their target.
The inclusion of the anti-ship missile systems came as British military intelligence said Russian naval forces were continuing to launch cruise missile strikes in support of ground operations in the region.
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It follows an urgent appeal by the Ukrainians for more weapons – including armoured vehicles – ahead of an expected Russian offensive on the eastern Donbas region, already part held by pro-Moscow separatists.
The PM said that while Russian forces attacking Kyiv had suffered a defeat, their withdrawal was "tactical" as they prepared to refocus their military efforts on the east.
He strongly condemned the "war crimes" being uncovered in the wake of their departure, with scores of bodies of civilians who have been shot and killed being reported by the Ukrainian authorities.
"I think what Putin has done in places like Bucha and Irpin, his war crimes have permanently polluted his reputation and the reputation of his government," he said.
Britain's Ministry of Defence (MOD) said the retreating Russian troops had left evidence of the "disproportionate targeting of non-combatants".
In one of its latest intelligence assessments, the MOD said this included "the presence of mass graves, the fatal use of hostages as human shields, and mining of civilian infrastructure".
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Volodymyr Zelensky welcomed the UK's "decisive and significant support" for his country, and urged other Western allies to intensify the pressure on Moscow.
"We have to exert pressure in the form of sanctions," the Ukrainian leader said.
"It is time to impose a complete embargo on Russian energy resources. They should increase the amount of weapons being supplied," he added.
As well as the additional equipment, Boris Johnson also confirmed further economic support, guaranteeing an additional $500m dollars (£385m) in World Bank lending to Ukraine, taking total UK loan guarantees to $1bn (£770m).
There were no immediate details of the armoured vehicles being sent. However, Defence Secretary Ben Wallace indicated during a visit to Romania on Friday that they would be Mastiff six-wheel-drive patrol vehicles.