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.

There has been no independent confirmation of the charges.

The Foreign Office said it condemned the exploitation of prisoners of war and civilians for political purposes.

"We are in constant contact with the government of Ukraine on their cases and are fully supportive of Ukraine in its efforts to get them released," added a spokesman.

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 to his children.

Described as a chef by training, Mr Healy is originally from Huntingdon in Cambridgeshire.

Dominik Byrne, co-founder of the Presidium Network, said his organisation had proof that Mr Healy was an independent humanitarian volunteer and was not involved in any military action.

His group has been in regular contact with the Healy family, who were "very scared" and wanted their son home as quickly as possible, he said.

The family was in contact with the British government and the Red Cross which was trying to get access to Mr Healy to check on his welfare, he added.

Mr Byrne said it was a horrible situation for the two men, who were likely being interrogated constantly and kept in awful conditions.

Russia's defence ministry has said Mr Hill was captured in the Mykolaiv region of south-west Ukraine. It is believed he was caught while fighting with Ukrainian forces.

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.

Last month, the family of Mr Aslin said he had been told that his execution would happen 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.

A third person - Moroccan man Brahim Saaudun, 21 - was tried alongside Mr Aslin and Mr Pinner and also sentenced to death.