Two Britons sentenced to death for fighting against Russian forces in Ukraine plan to appeal against the verdict.
The family of one of the men, Aiden Aslin, say he has been told his execution will be carried out.
The self-proclaimed Donetsk People’s Republic accused Mr Aslin, originally from Newark in Nottinghamshire, and Shaun Pinner from Bedfordshire of being foreign “mercenaries” after they were captured fighting for the Ukrainian army.
But their families as well as the UK government argue that they were legitimate members of the Ukrainian army and that they should, accordingly, be treated as prisoners of war under the Geneva Conventions.
The two men had lived in Ukraine since before the invasion.
The Foreign Office on Thursday said it is doing everything possible to support the two men.
But the family of Mr Aslin told the BBC they have spoken to him on the phone and say he has been warned “time is running out”.
“There are no words, just no words. It’s got to be everyone’s worst nightmare to have a member of your family threatened in this way,” said his grandmother, Pamela Hall.
“Aiden was extremely upset when he called his mother this morning. The bottom line is Aiden has said the [Donetsk People’s Republic] has told him nobody from the UK has made contact and that he will be executed.”
A lawyer for Mr Pinner, Yulia Tserkovnikova, told Russian news agency Tass that the verdict will be appealed.
British and Moroccan fighters captured by Russians in Ukraine sentenced to death
“My colleagues and I are working on the text of the appeal in the interests of our clients,” she said.
“If the appeal is rejected and the verdict comes into force, a request for a pardon will be filed because it is the inalienable right of our clients based on the laws of the Donetsk People’s Republic. It must not be violated and they strongly insist on using it.”
A Foreign Office representative told the PA news agency: “We are doing everything we can to support the men and are in close contact with and helping their families.
“We condemn the exploitation of prisoners of war for political purposes and have raised this with Russia.
“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.”
Downing Street has described the sentencing as a “sham judgment” with “absolutely no legitimacy”.
Asked on Thursday if there was any update on any kind of agreement to bring the men home, possibly via a prisoner swap, the representative said: “That would be a question for the Ukrainian government.
“As I say, they were fighting — as part of the Ukrainian armed forces and we continue to work closely with them. But again, we continue to call the judgment out for what it is, which is a sham.”