Downing Street has expressed alarm at reports British aid volunteer Paul Urey, who has family in Lancashire, has died while being detained by Russian-backed separatists in Ukraine.

Daria Morozova, the human rights ombudsperson for the Moscow-backed separatist leadership in Donetsk, said Paul Urey, who was branded a British “mercenary”, died in captivity on Sunday of chronic illnesses and stress.

Ms Morozova said: “From our side, he was given the necessary medical assistance despite the grave crimes he committed."

A No 10 spokesman said the Foreign Office is “urgently investigating” the “clearly alarming reports”, adding, "our thoughts are obviously with his family and friends."

In April, the Presidium Network, a non-profit group, said Mr Urey, who according to national media has family, including his mother, living in Preston, and Dylan Healey were captured at a checkpoint south of the city of Zaporizhzhia in south-east Ukraine.

Mr Urey's mother, Linda, has expressed her anger on Facebook, writing: “I’m truly angry.”

She said she had begged for the separatists to give her son back and branded them “murderers”, asking: “Why did you let him die?”

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’.”

Mr Urey, who was born in 1977 and from Culceth in Warrington, Cheshire, 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 evacuate.

The Presidium Network said it was concerned Russian forces may think the men are British spies.

The Foreign Office has said it is urgently seeking clarity about the situation.

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.”