Russia is "struggling with a crisis of medical combat provision," a UK intelligence update said.
Up to half of fatalities were preventable, and hospitals are likely being reserved for officers, it said.
Russia has averaged 400 casualties a day in the 17 months since it invaded Ukraine, the MOD said.
Russia is failing to help its injured soldiers, with up to 50% of Russian combat fatalities in Ukraine preventable, according to UK intelligence.
The UK Ministry of Defence said in an intelligence update on Monday that Russia has suffered an average of 400 casualties a day in the 17 months since it launched its invasion of Ukraine in February 2022.
As a result, "Russia is almost certainly struggling with a crisis of medical combat provision," it said.
The influx of casualties "has likely undermined the normal provision of some Russian civilian medical services, especially in border regions near Ukraine," the MOD added.
It also said that claims made by a Russian official that up to 50% of Russia's combat fatalities were preventable are likely true.
The MOD said the lack of medical help on the battlefield was having a devastating effect. "Very slow casualty evacuation, combined with the inappropriate use of the crude in-service Russian combat tourniquet, is reportedly a leading cause of preventable fatalities and amputations," it said.
It also said that it appeared likely "many dedicated military hospitals are being reserved for officer casualties."
The total number of Russia's losses in Ukraine is unclear, and Russia does not release its own figures.
A leaked US intelligence assessment said that up to 43,000 Russians were killed in action in the first year of the war — twice as many as the number of Ukrainians reportedly killed on the battlefield.
The MOD previously said that Russia had experienced an estimated 776 casualties a day in March when it was conducting its own offensive, but those numbers dropped to about 568 a day in April, as it moved to a more defensive posture, it said.
Russian news outlet The Insider, which has no connection to Insider, reported last month that hospitals where soldiers are being treated have supply shortages and rely on volunteers rather than help from Russian authorities.