The head of the UK's armed forces says Russia has already "strategically lost" the war in Ukraine and is now a "more diminished power".
Admiral Tony Radakin said Vladimir Putin had lost 25 per cent of Russia's land power for only "tiny" gains.
While Russia may achieve "tactical successes" in the coming weeks, he said any notion the war had been a success was nonsense.
Radakin, who is 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 Ukraine.
Radakin spoke to PA as he visited Scotland on Wednesday, where he met soldiers at Edinburgh Castle.
"This is a dreadful mistake by Russia. Russia will never take control of Ukraine," he said.
"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".
"The Russian machine is grinding away, and it's gaining a couple of - two, three, five - kilometres every day," 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."
Russia was a "more diminished power" diplomatically and economically than several months ago, Radakin said.
"Any notion that this is a success for Russia is nonsense. Russia is failing," he said.
"It might be getting some tactical successes over the last few weeks ... But Russia is losing strategically."
Radakin said he was in touch with his Ukrainian counterpart, General Valerii Zaluzhnyi, who recognised they were in a "tough battle" in the east.
Australian Associated Press
