Radakin said Russia’s troops were “in a mess” and the invasion was “not going well”, the Daily Mail reported.
His comments represent the most optimistic assessment yet of how the conflict may end, but it came on another bleak day, with Russian forces firing on families as they fled the fighting, the report said
When the invasion began on February 24, it was assumed to be inevitable that Russian tanks would roll into Kiev within hours.
But after a series of strategic blunders and the remarkable resistance of Ukrainian troops on the battlefield, the outcome of the campaign could now be in doubt.
Radakin, the former head of the Royal Navy who was appointed Chief of the Defence Staff late last year, was speaking after eight Russian aircraft were shot down in 24 hours.
The Russians, contrary to their military doctrine, have also been forced to admit that almost 500 of their soldiers have been killed.
And in a highly embarrassing example of ineptitude, a convoy of hundreds of Russian vehicles and an estimated 15,000 troops has ground to a halt, the Daily Mail reported.
The column, including tanks, missile batteries and armoured personnel carriers, had been earmarked by Putin to encircle Kiev and pound the Ukrainians into submission.
But this operation is considered at least a month behind schedule, according to UK military sources.
Given the unified approach to sanctions that the UK, US and other world powers have displayed, the Kremlin chief may not be able to sustain a military campaign for that long.
Asked on the BBC on Sunday whether Russia taking over Ukraine was “inevitable”, Radakin said: “No. I think we’ve seen a Russian invasion that is not going well.
“Russia is suffering, Russia is an isolated power. It is less powerful than it was 10 days ago. Some of the lead elements of Russian forces have been decimated by the Ukrainian response.
“You’ve also seen basic failures in terms of maintenance and their kit failing. Russia hasn’t operated at this scale since the Second World War and it is incredibly complex and difficult.”