Admiral Sir Tony Radakin, head of the British armed forces, says Russia now has a "more diminished" power and has already “strategically lost” the war in Ukraine.
Admiral Radakin said Russian President Vladimir Putin had lost 25 per cent of his land power for only “tiny” gains.
He said that while Russia may achieve “tactical successes” in coming weeks, any notion that the war had been a success was “nonsense”.
The UK’s Chief of Defence Staff said Russia was running out of troops and advanced missiles, and would never be able to take over all of Ukraine.
Meeting soldiers at Edinburgh Castle, Admiral Radakin said: “This is a dreadful mistake by Russia. Russia will never take control of Ukraine.
“Russia has strategically lost already. Nato is stronger. Finland and Sweden are looking to join.”
He said Russia had been forced to give up its objectives of taking over most Ukrainian cities and was now engaged in a tactical battle where fighting was “tough”.
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“The Russian machine is grinding away and it’s gaining a couple of — two, three, five — kilometres every day," Admiral Radakin said.
“And that’s tough for Ukraine, but this is going to be a long fight. And we’re supporting Ukraine, Ukraine has shown how courageous it really is.
“And Russia has vulnerabilities because it’s running out of people, it’s running out of high-tech missiles.
“President Putin has used about 25 per cent of his army’s power to gain a tiny amount of territory and 50,000 people either dead or injured."
Russia is now a “more diminished power” diplomatically and economically than several months ago, he said.
“Any notion that this is a success for Russia is nonsense. Russia is failing," Admiral Radakin said.
“It might be getting some tactical successes over the last few weeks. And those might continue for the next few weeks.
“But Russia is losing strategically.”
Admiral Radakin said he was in touch with his Ukrainian counterpart, Gen Valerii Zaluzhnyi, who recognised that they were in a “tough battle” in the east.
“They’re courageous people," Admiral Radakin said. "They’re ingenious people in terms of how they’re taking the fight to the Russians.
“But they’re also honest people, with saying actually they need some help.”
He said the UK would continue its support for Ukraine and “we’re in this for the long haul”.
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This week, Ukrainian presidential adviser Mykhailo Podolyak called on the West to help his country achieve “heavy weapons parity” with Russia in the war, calling for more heavy artillery, rocket systems and vehicles.
Admiral Radakin said he was “wary” of phrases like parity as no single capability would determine the outcome of the war.
The UK is providing MLRS rocket systems and has just delivered 120 armoured fighting vehicles, he said.
“We’ve been providing anti-tank weapons. There are other elements that we’re providing and that will continue," he said.
“But it’s the mass that’s provided by the whole of the international community — and it’s putting that alongside the courage and the resolve of the Ukrainian armed forces to fight for their territory.
“That is where the real parity lies, and where the real strength of Ukraine lies.”
