Paul Urey, 45, died in captivity in July, according to the human rights ombudsperson for the Moscow-supported leadership in Donetsk.
Daria Morozova, the ombudsperson, branded Mr Urey a “mercenary” and claimed he died in captivity of chronic illnesses and stress.
She added: “From our side, he was given the necessary medical assistance despite the grave crimes he committed."
But Mr Urey's body was returned with "signs of possible unspeakable torture”, according to Ukraine’s foreign minister Dmytro Kuleba.
He wrote on Twitter: “Russians have returned the body of a British humanitarian worker Paul Urie whom they captured in April and reported dead due to 'illnesses' and 'stress' in July. With signs of possible unspeakable torture.
“Detaining and torturing civilians is barbarism and a heinous war crime.
“I express my deepest condolences to relatives and close ones of Paul Urey. He was a brave man who dedicated himself to saving people. Ukraine will never forget him and his deeds.
“We will identify perpetrators of this crime and hold them to account. They won’t escape justice.”
After the news of his death, Mr Urey’s mum Linda Urey expressed her anger, branding the separatists “murderers”, and asking: “Why did you let him die?”
Russian ambassador to the UK Andrei Kelin was summoned to the Foreign Office to face questioning over what happened to Mr Urey, who was detained near the Ukrainian city of Zaporizhzhia in April.
Liz Truss, who was Foreign Secretary at the time, said she was “shocked” by reports of Mr Urey’s death.
She said in a statement at the time: “Russia must bear the full responsibility for this.
“Paul Urey was captured while undertaking humanitarian work. He was in Ukraine to try and help the Ukrainian people in the face of the unprovoked Russian invasion.
“The Russian government and its proxies are continuing to commit atrocities. Those responsible will be held to accountable. My thoughts are with Mr Urey’s family and friends at this horrendous time.”
Ms Urey said she was “truly angry” in a post on Facebook.
She said her son had been taken from her at birth and, after finding her, he had been taken from her again.
She added: “Cruel cruel, world."
In April, the Presidium Network, a non-profit group, said Mr Urey and fellow Brit Dylan Healey had been captured at a checkpoint south of the city in south-east Ukraine.
Mr Urey, who was born in 1977 and was from Manchester, and Mr Healey, born in 2000 and from Cambridgeshire, travelled to Ukraine of their own accord, the organisation said.
They were not working for the Presidium Network, which helps to get aid into Kyiv.
The organisation said the pair went missing while driving to help a woman and two children.
Paul Urey
Presidium Network
Vladimir Putin's Russia invaded Ukraine in February
Mikhail Metzel