The former PM has remained publicly quiet on Mr Sunak’s performance in recent months, despite taking on a highly-paid role as a columnist for the Daily Mail.
But in scathing remarks in a piece for The Spectator, the ousted Tory leader attacked his successor by saying the UK had given only a “fraction” of the arms that the US had provided to Kyiv.
“What the hell are we waiting for?” Mr Johnson asked. “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.”
He added: “Why are we always so slow? How can we look these men in the eye and explain the delay?”
Mr Johnson said that Ukraine needed more help with upgrading air defences, more drones, howitzer artillery weapons and Storm Shadow long-range cruise missiles.
The ex-PM also called on the US to provide ATACMs – the long-range missile systems still being withheld by Washington. He said the cost was “relatively trivial” and urged Mr Sunak and other western leaders to realise that the “stakes are enormous”.
“If Putin wins in Ukraine, if he holds even a fraction of what he has taken, then the lesson will be clear: that aggression pays, that European borders can once again be changed by violence where Putin fancies a revanchist and domestically rabble-rousing military operation,” he wrote.
Mr Johnson added: “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 ask it again: what the hell are we waiting for?”
He said western leaders “have underestimated the Ukrainians and overestimated Putin” since the invasion began in February 2021, adding that “we are doing the same today”.
Mr Johnson and his allies have been at odds with Mr Sunak’s government since he was kicked out of No 10 by his own party last summer.
The former PM clashed with ministers over the release of his WhatsApp messages to the Covid inquiry, and he was said to be furious at the denial of a peerage for Nadine Dorries in his resignation honours list.
Some of his remaining allies in the Commons have continued to snipe at Mr Sunak’s performance, but Mr Johnson has remained largely quiet about his successor.
He did openly criticise Mr Sunak’s post-Brexit deal with the EU to resolve the Northern Ireland protocol row – claiming it doesn’t “take back control” from Brussels.
No 10 said Mr Sunak considers the scandal over Partygate lies to be over after MPs’ decision to suspend Mr Johnson for 90 days and strip him of his parliamentary pass saw him quit his seat.