Briton Paul Urey, who was captured by Russian-backed separatists in Ukraine, has died in detention, the BBC has reported.
The UK's Foreign Secretary Liz Truss said Russia must bear full responsibility for his death.
The UK is seeking urgent clarification from Ukraine and Russia about the reports.
The Foreign, Commonwealth and Development Office (FCDO) said it had summoned the Russian ambassador, Andrey Kelin, to express its "deep concern".
Mr Urey, from Warrington, Cheshire, was detained at a checkpoint near the south-eastern city of Zaporizhzhia in April and accused of being a mercenary.
He was held captive in the self-proclaimed Donetsk People's Republic (DPR) along with another man, Dylan Healy, while reportedly trying to rescue a woman and her family trapped by the fighting.
The Presidium Network's Dominic Byrne told the BBC that Mr Urey had insulin on him when he was captured so his captors would have been aware of his diabetes condition.
Officials from the DPR said he died in captivity on 10 July from underlying health conditions and "stress".