A British Army commander who trained Ukrainian soldiers also said that Ukrainian troops and their commanders had struggled to adapt to Nato tactics and weapons.

“Despite them now receiving Western Nato weapon systems and ammunition, they have continued to refuse to adapt to Western tactics,” he told The Telegraph.

Russia’s invasion of Ukraine has become an attritional grind but, while Moscow has secured ammunition supplies from North Korea and Iran, Kyiv has complained that its Western allies have been slow to send promised shells and missiles.

Ukraine has also struggled to hit recruitment targets. It has given military recruiters more power and toughened laws but many young Ukrainian men now view a stint in the army as a one-way ticket to death or serious injury.

While Gen Marchenko’s comments on the “crumbling frontline” are rare from a Ukrainian commander, briefings from Western officials have become increasingly pessimistic about Ukraine’s chances of defeating Russia.

The US-based Institute for the Study of War, a pro-Ukraine think tank, said in its daily report on Wednesday that Russian forces had made “tactically significant” advances along the frontline in the past few weeks.