Dylan Healy, a chef who volunteered as a relief worker, was arrested at the checkpoint in April.
Later that day, Russia released a video of Andrew Hill wearing a military uniform, stating that he had surrendered.
This comes after two other British men, Shaun Pinner and Aiden Aslin, were sentenced to death last month.
They face charges similar of being mercenary to those of Mr. Healy and Mr. Hill in a Russian court case in eastern Ukraine, which is unknown worldwide.
The European Court of Human Rights (ECHR) has even stepped in to enforce the ban.
Russia, however, rejected the summons, saying it was no longer valid in Strasbourg court and that the men’s fate was the case in the Donetsk People’s Republic, which backed Russia.
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According to the Russian news agency Tass, an official who called himself the Donetsk People’s Republic said Mr Healy and Mr Hill would be prosecuted for “professional activities”. Both men reportedly refused to cooperate with investigators.
There was no independent verification of the charges of being mercenaries.
The Foreign Office said it condemned the exploitation of prisoners of war and civilians for political purposes.
“We are in constant contact with the Ukrainian government about their cases and fully support Ukraine in its efforts to secure their release,” a spokesman added.
Mr Healy’s abduction earlier this year was announced by a local aid organization, the Presidium Network, which said he and his British counterpart, Paul Urey, were conducting an independent humanitarian operation near Zaporizhzhia.
At the time of his arrest, Mr Healy was allegedly traveling with Mr Urey to rescue a family in the south of the city.
It is not yet clear where Mr Urey is now. In April, her mother, Linda, urged her captors to let her and her children come home.
Described as a professional chef, Mr. Healy hails from Huntingdon in Cambridgeshire.
Dominik Byrne, founder of the Presidium Network, said his organization had evidence that Mr. Healy was an independent volunteer and was not involved in any military action.
Her group has been in constant contact with the Healy family, who are “terrified” and want their son to return home immediately, he said.
The family had been in contact with the British government and the Red Cross trying to get Mr Healy to assess his welfare, he added.
Mr Byrne said it was a traumatic situation for the two men, who may have been interrogated regularly and kept in critical condition.
Russia’s defense ministry said Mr Hill had been detained in the Mykolaiv region in southwestern Ukraine. He is believed to have been caught fighting with Ukrainian troops.
Wearing a mask in a Russian video, with his arm wrapped in a sling, he said he was from Plymouth and had four children with his partner. He said he went out of his way to help Ukraine.
Last month Mr Aslin’s family said he had been told his assassination would happen and time was running out.
“No words – it must be the worst nightmare,” Mr Aslin’s grandmother Pamela Hall told the BBC. They couldn’t be mercenaries.
The UK government – which has called death sentences “a fraudulent sentence with no merit” – said Mr Aslin and Mr Pinner should be treated as prisoners of war under the rules set out in the Geneva Conventions.
He said he was doing his best to support the men and was in close contact with their families.
A third person – Moroccan man Brahim Saaudun, 21 – was tried and Mr Aslin and Mr Pinner were sentenced to death on accusation of being mercenaries.