Boris Johnson formed a strong bond with the Ukrainian president.
The former prime minister said the UK and other western nations were guilty of “dragging our feet” as the war continues.
Johnson’s comments echo his calls last February for the UK to provide Kyiv with fighter jets.
Writing in The Spectator, he said there was no chance of Ukraine negotiating with Vladimir Putin to bring an end to the conflict, so they must be given as much help as possible to defeat him on the battlefield.
He said: “There is only one thing they want from us, and that is the weaponry to finish the job – and so I simply do not understand why we keep dragging our feet.
“Why are we always so slow? How can we look these men in the eye, and explain the delay? Throughout this war we have underestimated the Ukrainians and overestimated Putin, and we are doing the same today.”
Johnson added: “From the UK, they are hoping for more help with howitzers, with Stormshadow, with air defences, and they want as much help as we can give on drone technology.”
Accusing America of also failing to give Ukraine what it needs, he went on: “We are talking about a relatively trivial outlay for such extraordinary potential reward.
“The US has given only about 1 per cent of its annual defence budget to support Ukraine’s armed forces, and the UK has given a fraction of what the US has given.”
Johnson said Putin had “intensified and provoked the most powerful modern nationalism we have seen” among the Ukrainian population.
“His troops, tired, mistrustful, far from home, have nothing in their hearts to match it,” he said.
″That is why Putin will lose and Ukraine will eventually win; and since that is what must happen, and since that is what is going to happen, can we not, in the name of all that is holy, give the Ukrainians now the military assistance they need to bring matters to the speediest possible conclusion so that fewer human beings are thrown into the charnel-house of this pointless conflict?
“I have asked it before, and I ask it again: what the hell are we waiting for?”
The criticism will come as blow to Sunak, who has been eager to shake off the lingering presence of his predecessor.