The Chief of the Defence Staff says the military threat from Russia could last decades.
Admiral Sir Tony Radakin told BBC One's Sunday Morning show "the challenge of Russia is going to endure way beyond 2022 and 2023 and 2024".
The former First Sea Lord said speculation that Russian President Vladimir Putin is not well or could be assassinated is just "wishful thinking".
He added: "For us, as military professionals, we see a relatively stable regime in Russia, we see that President Putin has been able to quash any opposition and then we see a hierarchy that is invested in President Putin and so nobody at the top has got the motivation to challenge President Putin, and that is bleak."
Ukraine’s army "absolutely" believes it will win the war against "struggling" Russia which has "lost more than 30% of its land combat effectiveness", Admiral Radakin said.
"Russia started this invasion with the ambition to take the whole of Ukraine. Russia had the ambition to take the cities in the first 30 days.
"Russia had the ambition to create fractures and to apply pressure to NATO. This is Russia as a challenge to the world order.
"Russia is failing in all of those ambitions. Russia is a more diminished nation than it was at the beginning of February," he added.