Admiral Sir Tony Radakin said Vladimir Putin had lost 25% of Russia’s land power for only “tiny” gains.
While Russia may achieve “tactical successes” in the coming weeks, he said any notion that the war had been a success was “nonsense”.
Admiral 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 of Ukraine.
He 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 added: “The Russian machine is grinding away, and it’s gaining a couple of – two, three, five – kilometres every day.
“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.”
He continued: “President Putin has used about 25% 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.
The chief of defence staff continued: “Any notion that this is a success for Russia is nonsense. Russia is failing.
“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.”
According to the latest update from the Ministry of Defence (MoD) Russia’s war in Ukraine has likely accelerated the state’s long-term trajectory towards authoritarianism.
The update from the MoD reads: “Over the last 24 hours, Russian forces have likely continued to attempt to regain momentum on the Popasna axis, from which they seek to surround the Sieverodonetsk pocket from the south.
"In Russia, the war has accelerated the state’s long-term trajectory towards authoritarianism. In recent weeks, the Duma has started the process to introduce a 20-year sentence for Russians who fight against the Russian Federation.
"Speaking out against the invasion is also being criminalised.
Despite the majority of Russians telling pollsters they support the ‘special military operation’, elements of the population both actively and passively demonstrate their opposition."
It added: “Freedom for Russia Legion”, recruited from Russians, has almost certainly deployed in combat alongside the Ukrainian military. Some high profile Russian officials have highly likely been side-lined after criticising the war.
"Scepticism about the war is likely also particularly strong amongst Russia’s business elite and oligarch community. Migration applications suggest that 15,000 Russian millionaires (in US dollars) are likely already attempting to leave the country.
"Motivations highly likely include both personal opposition to the invasion and an intent to escape the financial impact of the sanctions imposed on Russia. Should this exodus continue, it will likely exacerbate the war’s long-term damage to Russia’s economy.”