Paul Urey, from Manchester, travelled to the war-torn country to undertake aid missions of his own accord.
He had been captured alongside 22-year-old Dylan Healey.
Seperatist rebels have now claimed that Mr Urey died on Sunday while he was still detained by Russian forces.
What happened to Paul Urey?
Mr Urey had travelled to Ukraine to take part in aid missions, without being tied to any organisations.
Both Mr Urey and Mr Healey are said to have been helping to evacuate a woman and two children when they were captured at a checkpoint south of the city of Zaporizhzhia by Russian forces at the end of April.
Russia branded the pair “mercenaries” upon their capture.
The British Foreign Office had been working to establish the status of Mr Urey, Mr Healey and other aid workers who had been captured by Russian forces while attempting aid missions in Ukraine throughout their detainment.
However, seperatist rebels have now said that Mr Urey has passed away.
How did Paul Urey die?
The Moscow-backed separatist leadership in Donetsk has said that Mr Urey died due to a mixture of chronic illness and stress.
Daria Morozova, the human rights ombudsperson for the group, said: “From our side, he was given the necessary medical assistance despite the grave crimes he committed.”