Paul Urey, 45, died in captivity on Sunday, according to the human rights ombudsperson for the Moscow-supported leadership in Donetsk.
He was detained and charged with “mercenary activities” by separatists at a checkpoint near Zaporizhzhia, in south-eastern Ukraine in April.
His mother Linda Urey expressed her anger, branding the separatists “murderers”, and asking: “Why did you let him die?”
Now Foreign Secretary Liz Truss has summoned the ambassador amid concerns Mr Urey died while being detained by Russian-backed separatists.
Responding to Mr Urey’s reported death, Ms Truss said in a statement: “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. 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.”
An official in the self-proclaimed Donetsk People's Republic (DPR), a breakaway entity recognised only by Russia, Syria and North Koeria, earlier said Mr Urey had died.
Daria Morozova, the ombudsperson, branded Mr Urey a “mercenary” and claimed he 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.
Writing on Facebook, his mother, from Preston, said she was “absolutely devastated” and “truly angry”.
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’.”
In April, the Presidium Network, a non-profit group, said Mr Urey and fellow Briton Dylan Healey, 22, 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.
The Presidium Network said it was concerned that Russian forces might think the men were British spies.
The group described Mr Urey as a humanitarian worker and denied he had any military background.