Foreign Secretary Liz Truss is to raise with her Ukrainian counterpart the case of two Britons sentenced to death for fighting Russian forces as the Government comes under pressure to ensure their release.

The Cabinet minister will discuss the “sham judgment” issued by a Russian proxy court against Aiden Aslin, 28, and Shaun Pinner, 48, during a phone call with Dmytro Kuleba on Friday.

Mr Aslin’s family urged Britain and Ukraine to “do everything in their power to have them returned to us safely, and soon” as they insisted the two men “are not, and never were, mercenaries”.

The pair, who were fighting with Kyiv’s forces, were convicted of taking action towards violent seizure of power at a court in the self-proclaimed Donetsk People’s Republic.

Ms Truss said: “I utterly condemn the sentencing of Aiden Aslin and Shaun Pinner held by Russian proxies in eastern Ukraine.

“They are prisoners of war. This is a sham judgment with absolutely no legitimacy.

“My thoughts are with the families. We continue to do everything we can to support them.”

It is understood that the call with Mr Kuleba had been scheduled for a number of days, to discuss the UK’s support of Ukraine, but that Ms Truss will raise the case of the two detainees as Britain treads a diplomatic tightrope.

There were concerns 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, from Newark in Nottinghamshire, and Mr Pinner, from Bedfordshire, are members of the Ukrainian army and should therefore be treated as prisoners of war.

In a statement to the Newark Advertiser, Mr Aslin’s family said: “We love Aiden with all our hearts. He and Shaun, as members of Ukrainian armed forces, should be treated with respect, just like any other prisoners of war.

“They are not, and never were, mercenaries.

“We hope that this sentence will be overturned and beseech the governments of the UK and Ukraine to do everything in their power to have them returned to us safely, and soon.

“We can only imagine what they are going through right now.

“This is a very upsetting development and we ask that our privacy is respected at this time.”

Downing Street said it is “deeply concerned” by the situation.

“We have said continually that prisoners of war shouldn’t be exploited for political purposes,” a spokesman said.

“Under the Geneva Convention, prisoners of war are entitled to combatant immunity and they should not be prosecuted for participation in hostilities.

“So we will continue to work with the Ukrainian authorities to try to secure the release of any British nationals who were serving in the Ukrainian armed forces and who are being held as prisoners of war.”

Government minister Robin Walker said dialogue with Russia is also needed after the “sham trial”.

He told Times Radio: “I think we need to do everything we can at a diplomatic level to make representations to Russia, to show our support to Ukraine, but also to show our support to the families of these people, and I understand that’s what my colleagues at the Foreign Office are doing.

“We have to be very clear who is responsible for this – both for the illegal, unprovoked invasion of Ukraine and for this sham trial by a government which doesn’t have authority, and that’s of course Russia.

“Russia needs to take responsibility, its responsibilities under the Geneva Convention, for the treatment of prisoners of war.”

A person walks down a quiet street with closed shops in Newark-on-Trent
Aiden Aslin is originally from Newark-on-Trent in Nottinghamshire (Mike Egerton/PA)

A third man, Moroccan national Saaudun Brahim, was convicted alongside Mr Aslin and Mr Pinner.

The men were accused of being “mercenaries” after fighting with Ukrainian troops.

Interfax, a Russian news agency, claimed the men would be able to appeal against their convictions.

Mr Aslin and Mr Pinner were both members of regular Ukrainian military units fighting in Mariupol, the southern port city which was the scene of some of the heaviest fighting since Vladimir Putin’s invasion of Ukraine.

Tory former minister Robert Jenrick, who represents the constituency where Mr Aslin lived, called for the Russian ambassador to the UK to be summoned to the Foreign Office.

It comes after a friend of Mr Aslin said the death sentences will “invigorate” those still resisting Russia’s advances.