A British aid worker who was detained by Russian separatists in Ukraine has died, a family member has told Sky News.
Paul Urey is believed to have died five days ago "due to sickness", according to information his family has received from the Foreign Office.
He was charged with committing "mercenary activities" after being captured in the self-proclaimed Donetsk People's Republic (DPR) in eastern Ukraine.
Following the news, The Foreign, Commonwealth & Development Office (FCDO) summoned the Russian Ambassador, Andrey Kelin.
Foreign Secretary Liz Truss said: "I am shocked to hear reports of the death of British aid worker Paul Urey while in the custody of a Russian proxy in Ukraine.
"Russia must bear the full responsibility for this.
"The Russian government and its proxies are continuing to commit atrocities. Those responsible will be held accountable."
Mr Urey, 45, was seized at a checkpoint outside the southeastern city of Zaporizhzhia on 25 April alongside Dylan Healy, 22, a fellow British volunteer.
Daria Morozova, who has the title of human rights ombudsman in the DPR, said on social media that Mr Urey had been suffering from diabetes and respiratory, kidney and cardiovascular issues.
She said: "On our part, despite the severity of the alleged crime, Paul Urey was provided with appropriate medical assistance. However, given the diagnoses and stress, he passed away on July 10."
In response, Mr Urey's mother Linda Urey said in a post online that she is "absolutely devastated".
The Foreign Office condemned the exploitation of prisoners of war after Mr Healy and fellow Briton Andrew Hill were held by Kremlin forces in eastern Ukraine.
Mr Urey's daughters had told Sky News they were preparing for the worst after they found out that he had been detained in April.
They said they did not know he was in Ukraine until they learnt of his capture.
After Mr Urey was seized, he was shown handcuffed on Russian TV.
During the interview, Mr Urey claimed he wanted to visit Ukraine "to see if it was as bad as the news said" but it is not known if he was speaking under duress.
Ms Urey denounced the interview and said while it was her son in the interview he was "not acting in his natural way".
Two other Britons and a Moroccan man who were captured while fighting for Ukraine have been sentenced to death in the DPR for mercenary activities.
The DPR is a breakaway entity which is recognised only by Russia, Syria and North Korea.