Britain's chief of defense staff, Admiral Sir Tony Radakin, said Russia had "strategically lost" the war in Ukraine and had been "further weakened".
Although Russia may achieve "tactical success" in the coming weeks, he said any idea that Russia might succeed in this war is "nonsense."
His comments came as Defense Secretary Ben Wallace told the media that it was "highly likely" that Britain would send hundreds of additional troops to NATO forces in Estonia to further contain Russia.
Sir Tony said: "This is a terrible mistake by Russia. Russia will never be able to control Ukraine.
"Russia has failed strategically. NATO is becoming more powerful, Finland and Sweden are looking to join.
He said Moscow had been forced to abandon its goal of taking over most of Ukraine's cities and was now fighting a "difficult" tactical battle.
Sir Tony said: "The Russian machine is struggling and it is adding two or three more every day." , five kilometers.
"It will be difficult for Ukraine, but it will be a long fight. We support Ukraine, which has proven its mettle.
He continued: "The PT used 25% of his army to gain a small amount of territory and 50,000 Russian servicemen were killed or wounded.
He said Russia was now a "dangerously weakened state" both diplomatically and economically compared to just a few months ago.
Any idea that this would be a success for Russia is nonsense. Russia is undergoing Failed.
" It may have had some tactical success over the past few weeks. These are likely to continue over the next few weeks.
"But Russia is losing strategically."
The head of Britain's armed forces said he was in touch with his Ukrainian counterpart, General Valery Zaruzhny, who admitted they were in a "hard battle" in the east.
The defense minister admitted on Thursday that Russia outnumbered Ukraine by 20 to one in some areas.
But Mr Wallace told Sky 's defense and security correspondent Deborah Haines that as NATO began supplying Ukraine with long-range artillery and rocket systems they would soon be able to make "significant progress in the east of the country" .
Sir Tony pays tribute to Ukrainians . "They are brave people," he said. "They are smart people in terms of how to fight the Russians.
"But they are also honest people and say they actually need some help.
He said the UK would continue to support Ukraine and "we are committed to this for the long term".
Earlier this week, Ukrainian presidential adviser Mykhailo Podolyak urged Western countries to help Ukraine in the war Achieving "heavy weapons parity" with Russia, calling for a significant increase in Western aid for heavy artillery , multiple launch rocket systems and armored vehicles Sir Tony said he was "cautious" about such claims. Because no one capability can determine the outcome of a war.
He said the UK is supplying MLRS rocket systems and recently delivered 120
"We have been supplying anti-tank weapons and we are also supplying other capabilities. things that helped Ukraine win and will continue to provide".
"But these things that the entire international community has to offer can only be compared with the courage and determination of the Ukrainian armed forces fighting for their territory.
“This is where true reciprocity lies and where Ukraine’s real strength lies.”
