Boris Johnson has condemned the “sham” trial of two Britons sentenced to death by the Russian authorities for fighting in Ukraine, as the government comes under pressure to ensure their release.

       Downing Street said the prime minister was “appalled” at the sentencing of Aiden Aslin and Shaun Pinner, and vowed that the government would do “everything in its power” to reunite them with families.

       “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,” said Mr Johnson’s official spokesman.

       They added: “[Mr Johnson] 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.”

       Foreign secretary Liz Truss discussed the verdict issued by a proxy court in Russian-controlled east Ukraine during a phone call with her Ukrainian counterpart Dmytro Kuleba on Friday.

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       After the call, she posted on social media: “The judgement against them is an egregious breach of the Geneva convention. The UK continues to back Ukraine against Putin’s barbaric invasion.”

       However, No 10 declined to say whether the Russian ambassador to the UK, Andrei Kelin, would be summoned to the Foreign Office over the issue, or whether representations would be made to the Kremlin.

       “We don’t have regular interaction with the Russians,” they added, stressing the government’s priority is working with Ukrainian counterparts to “ensure their release as quickly as possible”.

       “They’re afforded protection under the Geneva Convention as members of the Ukrainian Armed Forces, which is why we want to continue working with them closely to try and get them freed as quickly as we can.”

       There are concerns in the Foreign Office that making their case a bilateral issue between the UK and Russia would assist Moscow in its narrative that the men are “mercenaries” and therefore not entitled to protection under international law.

       Britain argues that Mr Aslin (28), from Nottinghamshire, and Mr Pinner (48), from Bedfordshire, are members of the Ukrainian army and should therefore be treated as prisoners of war.

       Speaking on Friday, the Russian foreign minister Sergey Lavrov declined to comment on the case, saying: “Currently they are guided by the laws of the Donetsk People’s Republic”.