Andrew Hill and aid worker Dylan Healy are understood to be under investigation as mercenaries.
Two more Britons captured by Russian forces in Ukraine have been charged with being mercenaries, according to Russian state media.
Dylan Healy, a chef who was volunteering as an aid worker, had been captured at a checkpoint in April.
On the same day, Russia released a video of Andrew Hill in military uniform, saying he had surrendered.
It comes after two other British men, Shaun Pinner and Aiden Aslin, were sentenced to death last month.
They faced the same charges as Mr Healy and Mr Hill in the hearing at a Russian proxy court in eastern Ukraine, which is not internationally recognised.
The European Court of Human Rights has since intervened to demand the sentence is not carried out.
But Russia has rejected the call, saying it no longer implements the Strasbourg court’s decisions and the fate of the men was a matter for the pro-Russian, breakaway Donetsk People’s Republic.
According to a report by Russia’s Tass news agency, an anonymous official from the self-styled Donetsk People’s Republic claimed Mr Healy and Mr Hill would stand trial for “mercenary activities”.
It reported both men were refusing to co-operate with investigators.
Mr Healy’s capture earlier this year was publicised by an aid organisation working in the area, the Presidium Network, which said he and another Briton, Paul Urey, had been carrying out humanitarian work independently near Zaporizhzhia.
At the time of their capture, Mr Healy was said to be driving with Mr Urey to rescue a family from a village south of the city.
It is unclear where Mr Urey is now. In April, his mother, Linda, urged his captors to let him come home to her and his children.
Dominik Byrne, co-founder of Presidium Network, said his group was in regular contact with the Healy family, who were “very scared” and wanted their son home as quickly as possible.
He said the family had been in contact with the British government and the Red Cross which was trying to get access to Mr Healy to check on his welfare.
Mr Byrne said it was a horrible situation for the two men, who were likely being interrogated constantly and kept in awful conditions.
He said Mr Healy had no connection to the Ukraine military or any foreign legions and was in the country as a volunteer.
Described as a chef by training, Mr Healy is originally from Huntingdon in Cambridgeshire.
Russia’s defence ministry has said Andrew Hill had been captured in the Mykolaiv region of southwestern Ukraine.
Wearing camouflage in the video released by Russia, and with his arm in a sling, he said he was from Plymouth and had four children with his partner. He said he had travelled of his own accord to help Ukraine.
Earlier this month, the family of Mr Aslin said he had been told that the execution would be carried out and time was running out.
“There are no words – it’s got to be everyone’s worst nightmare,” Mr Aslin’s grandmother Pamela Hall told the BBC.
The UK government – which has called the death sentences a “sham judgement with absolutely no legitimacy” – has said Mr Aslin and Mr Pinner should be treated as prisoners of war under the laws laid out in the Geneva Conventions.
It said it was doing everything it could to support the men and was in close contact with their families.
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