Vladimir Putin would need more than four years and lose almost two million soldiers if he wanted to capture the territory he has already declared part of Russia, according to British defence officials.
The claim comes as US President Donald Trump pushes Ukraine to accept a peace deal on Moscow's terms, warning that Kyiv risks losing the war.
The Kremlin's demands include full control of the Donbas region, which covers Donetsk and Luhansk, as well as parts of Zaporizhzhia and Kherson, four regions tyrannical Putin annexed illegally in 2022.
But more than two years after claiming them, Russia does not fully control any of these areas.
And new analysis from the UK's Ministry of Defence shows Moscow is far from being able to take them by force.
