Two Britons and a Moroccan who were captured while fighting for Ukraine were sentenced to death on Thursday by a court in the self-proclaimed Donetsk People’s Republic (DPR), one of Russia’s proxies in eastern Ukraine, Russian news agencies reported.

The court found the three men — Britons Aiden Aslin and Shaun Pinner and Moroccan Brahim Saadoun — guilty of “mercenary activities and committing actions aimed at seizing power and overthrowing the constitutional order of the DPR”, the Interfax news agency quoted a court official as saying.

The three men were captured while fighting for Ukraine against Russia and Russian-backed forces after Russia invaded on February 24. Their lawyer said they would appeal against the decision. Britain slammed the court’s decision as a “sham judgment”.

“I utterly condemn the sentencing of Aiden Aslin and Shaun Pinner held by Russian proxies in eastern Ukraine,” British foreign secretary Liz Truss said on Twitter.

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

Prime Minister Boris Johnson’s spokesman said that, under the Geneva Conventions, prisoners of war were entitled to combatant immunity and they should not be prosecuted for participation in hostilities.

Robert Jenrick, the MP for the district where Aslin’s family live, said the proceedings were akin to a “Soviet-era show trial” and demanded Truss summon the Russian ambassador to the foreign office.

“The Russian ambassador and the Russian government need to know they won’t get away with this,” he told the BBC.

During the proceedings, the three men were held in a cage with black bars, guarded by soldiers with their faces covered and wearing arm-bands with the pro-Russian “Z” symbol, before being asked to stand while the verdict was read to them, a video from the courtroom published by the RIA Novosti news agency showed.

The hasty trial was held largely behind closed doors, with information on proceedings handed to select state-owned Russian media agencies. Less than 24 hours before the verdict was handed down, Pinner and Saadoun had pleaded guilty to actions aimed at the violent seizure of power, a video shared from the court by the RIA Novosti news agency showed.

Aslin appeared to have pleaded guilty to a lesser charge involving weapons and explosives.

“The evidence presented by the prosecution in this case allowed the court to pass a guilty verdict, not to mention the fact that all the defendants, without exception, pleaded guilty to all charges,” judge Alexander Nikulin told reporters at the court after handing down the verdict.

“When passing the verdict, the court was guided not only by the prescribed norms and rules, but also by the most important, unshakable principle of justice. It was that which made it possible to take this complex and difficult decision to apply an exceptional measure of punishment in the form of the death penalty,” he added.

The DPR is one of two breakaway Russian-backed entities in the Donbas region of eastern Ukraine that Russia says it is fighting to liberate from Ukrainian forces.

Three days before launching its February 24 invasion of Ukraine, Russia recognised them as independent states in a move condemned by Ukraine and the west as illegal.