Briton Paul Urey, who was captured by Russian-backed separatists in Ukraine, has died in detention, reports say.
The 45-year-old’s family were contacted by the UK Foreign Office, says the Presidium Network, an aid group which has been helping his relatives.
His mother is “distraught and still in shock”, the group says.
The UK says it is seeking urgent clarification from Ukraine and the Russian government on reports of a British aid worker’s death.
“We are urgently raising this with our Ukrainian allies and the Russian Ministry of Foreign Affairs,” the UK’s Foreign, Commonwealth and Development Office said.
“We continue to be in close contact with the family.”
Mr Urey, from Warrington, Cheshire, was detained at a checkpoint near the south-eastern city of Zaporizhzhiain 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.
Mr Urey’s mother had said she was extremely worried for his welfare, because he had type 1 diabetes and needed insulin.
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 condition.
Officials from the DPR, who were holding Mr Urey, say he died in captivity on 10 July from underlying health conditions and “stress”.
A Downing Street spokesman said: “They’re clearly alarming reports and our thoughts are obviously with his family and friends.”
A DPR official said British officials had been informed of Mr Urey’s imprisonment but there had been “no reaction from the UK”.
According to the DPR’s Daria Morozova, during Mr Urey’s first medical examination in the DPR, he was found to have insulin-dependant diabetes, damage to the respiratory system and kidneys, and a number of diseases of the cardiovascular system.
He was also in a “depressed psychological state due to indifference to his fate in his homeland”, Ms Morozova said.
She said the UK had not provided the necessary medical supplies through the International Committee of the Red Cross.
“For our part, despite the severity of the alleged crime, Paul Urey was provided with appropriate medical care,” she said.
Mr Byrne, however, said that Mr Urey’s family blamed the Russian government and his captors for his death.
The captive had been denied visits from agencies like the Red Cross by the Donetsk and Russian authorities, he added.
“Because of that it was really showing that he wasn’t looked after properly and was never allowed to be seen,” Mr Byrne said.
He added that the chances of Mr Uley’s body being returned to his family were “low”.
Presidium Network said Mr Urey had previously spent eight years as a civilian contractor in Afghanistan. He had been living in Leyland, Lancashire, prior to travelling to Ukraine.