Anger was growing on Friday over the fate of two Britons sentenced to death by a “kangaroo court” in occupied Ukraine.

Aiden Aslin, 28, and Shaun Pinner, 48, were captured while fighting for Ukrainian forces and convicted on Thursday of carrying out terrorist acts at a court in the self-proclaimed Donetsk People’s Republic, the eastern region of Ukraine now at the centre of the war.

But while Foreign Secretary Liz Truss condemned the ruling as a “sham judgment”, Foreign Office officials are reluctant to escalate a diplomatic row with Russia for fear of strengthening its claim that the two men are foreign mercenaries.

Britain argues that the two men, who have family in Ukraine, are members of the Ukrainian army and should therefore be treated as prisoners of war.

Ministers hope that they will be able to work with Ukraine to secure their release through a prisoner swap with Russia.

Ms Truss raised the case of the prisoners in a call with Ukraine’s foreign minister Dmytro Kuleba on Friday.

He said: “What is uppermost in her mind is access to proper health services so they can assure Shaun and Aiden are well and there’s been no coercion.

“And that they have access to independent legal advice as they seek to prepare their defence against this sham trial and this judgment which is completely against international law.”

Following the judgment by the Donetsk court yesterday the three men were told they have one month to appeal and possibly receive a reduced sentence.

The governor of the Luhansk region of Ukraine, Serhiy Haidai, said that the Britons would have signed official documentation upon joining the Ukrainian armed forces and that the verdict is therefore “not applicable to them, even if they are not Ukrainians”.