The family of a British man condemned to death for fighting Russian forces say they are "devastated".
Foreign Secretary Liz Truss discussed efforts to secure the release of Aiden Aslin and Shaun Pinner with her Ukrainian counterpart on Friday, after the judgment by a Russian proxy court.
Prime Minister Boris Johnson has ordered ministers to do "everything in their power" to secure the release of the two Brits after they were condemned to death for fighting Russian forces.
A statement now issued by the Foreign Office on behalf of the family of Shaun Pinner says: "Firstly, our whole family is devastated and saddened at the outcome of the illegal show trial by the so-called Donetsk People's Republic.
"As a Ukrainian resident for over four years and contracted serving marine in the 36th Brigade, of which he is very proud, Shaun should be accorded all the rights of a prisoner of war according to the Geneva Convention and including full independent legal representation.
"We sincerely hope that all parties will co-operate urgently to ensure the safe release or exchange of Shaun.
"Our family including his son and Ukrainian wife love and miss him so much and our hearts go out to all the families involved in this awful situation.
"We respectfully ask for privacy from the media at this difficult time."
British citizens Aiden Aslin (L) and Shaun Pinner (R) and Moroccan Saaudun Brahim (C) ( Image: STRINGER/EPA-EFE/REX/Shutterstock)
Ukraine's ambassador to the UK, Vadym Prystaiko, suggested negotiations for a possible prisoner swap with Moscow were under way, as it emerged Defence Secretary Ben Wallace made a surprise visit to Kyiv for talks with Ukrainian President Volodymyr Zelensky.
A relative of Mr Aslin urged Britain and Ukraine to "do everything in their power to have them returned to us safely, and soon".
They said Mr Aslin, 28, and Mr Pinner, 48, "are not, and never were, mercenaries" and should be treated as prisoners of war, as they were fighting as part of the Ukrainian army.
The men were convicted of taking action towards violent seizure of power, at a court in the self-proclaimed Donetsk People's Republic.
Aiden Aslin (L), Shaun Pinner (R) and Saaudun Brahim (C) ( Image: STRINGER/EPA-EFE/REX/Shutterstock)
A No 10 spokesman said: "The Prime Minister was appalled at the sentencing of these men.
"He has been following the case closely and has asked ministers to do everything in their power to try and reunite them with their families as soon as we can.
"We completely condemn the sham sentencing of these men to death. There's no justification at all for this breach of the protection they're entitled to."
Ms Truss said she discussed "efforts to secure the release of prisoners of war held by Russian proxies" during her call with Dmytro Kuleba, Ukraine's minister of foreign affairs.