The UK Ministry of Defence says the death toll from a Ukrainian attack on a building that killed a large number of Russian soldiers in the occupied town of Makiivka was likely exacerbated by storage of ammunition there.
The building was completely destroyed and Russian officials confirmed that 89 Russian personnel were killed.
"Given the extent of the damage, there is a realistic possibility that ammunition was being stored near to troop accommodation, which detonated during the strike creating secondary explosions," said the UK report.
"The Russian military has a record of unsafe ammunition storage from well before the current war, but this incident highlights how unprofessional practices contribute to Russia's high casualty rate," the UK ministry added.
The building was hit when Ukrainian forces fired six rockets from a US-provided HIMARS multiple-launch system.
A former pro-Russian separatist commander had previously also said the fatalities and injuries were likely worse because ammunition was in storage at the former vocational school building.
He also said Russia was likely downplaying the number of deaths at the site.
Ukraine's General Staff said on Tuesday its forces had killed some 500 mobilized Russian soldiers when it hit the building.