Paul Urey was reported to have died in captivity on Sunday.
She said she had begged for the separatists to give her son back and branded them “murderers”, asking: “Why did you let him die?”
Cruel cruel world
Paul Urey's mother Linda
In a post, she said her son had been taken from her at birth and, after finding her, he had been taken from her again.
“Cruel cruel world,” she added.
Speaking at the time of his capture, Ms Urey told Sky News she had begged her son not to go to Ukraine.
She added: “He said, ‘Muma I can’t live with myself knowing people… need help to get to a safe place, I have to go. I would feel bad’.”
Daria Morozova, the human rights ombudsperson for the Moscow-backed separatist leadership in Donetsk, said Mr Urey, who was branded a British “mercenary”, died in captivity on Sunday of chronic illnesses and stress.
“From our side, he was given the necessary medical assistance despite the grave crimes he committed,” she added.
A No 10 spokesman said the Foreign Office is “urgently investigating” the “clearly alarming reports”.
BREAKING. British volunteer Paul Urey "has died" in prison in Donetsk due to "chronic illnesses", Donbas pro-Russian separatists announce.
— Antonello Guerrera (@antoguerrera) July 15, 2022
“Our thoughts are obviously with his family and friends,” he added.
In April, the Presidium Network, a non-profit group, said Mr Urey and Dylan Healey had been captured at a checkpoint south of the city of Zaporizhzhia 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.
The Presidium Network said it was concerned that Russian forces might think the men were British spies.
A Foreign, Commonwealth and Development Office spokesperson said: “We are urgently seeking clarification from the Russian government on media reports that a British aid worker has died in Ukraine.”