Two British men and a Moroccan national captured while fighting in the Ukrainian army in Mariupol have been sentenced to death by pro-Russia officials after a days-long process described as a “disgusting Soviet-era show trial”.

A court in Russian-controlled east Ukraine convicted 28-year-old Aiden Aslin, from Newark, 48-year-old Shaun Pinner, from Watford, and Saaudun Brahim on charges of “terrorism”. Observers said the process was intended to imitate the war crimes trials of Russian soldiers taking place in Kyiv.

Both Britons have said they were serving in the Ukrainian marines, making them active-duty soldiers who should be protected by the Geneva conventions on prisoners of war. However, Russian state media has portrayed them as mercenaries, and the court has convicted them on the charge of “being a mercenary”.

The ruling was swiftly condemned by top British officials.

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

On Wednesday, the state-run news agency RIA Novosti shared footage of the men pleading “guilty” to the charges against them, which also included terrorism, committing a crime as part of a criminal group, and forcible seizure of power or forcible retention of power.

The pro-Russian officials claimed the men’s actions had “led to the deaths and injury of civilians, as well as damage to civilian and social infrastructure”.

A pro-Russia official in Donetsk said that while the men had pleaded guilty, they were given the death penalty because of “the main, unshakable principle – justice”. The process was closed and only small fragments of the proceedings were made public through pro-Kremlin media.

An official said that the men would have one month to appeal against their sentence and, if an appeal was accepted, they could receive a life or 25-year prison sentence instead of the death penalty.

Russia is believed to be using the process in part to put pressure on the UK and may seek a prisoner exchange for Russian soldiers convicted of murder and other war crimes during Russia’s invasion of Ukraine. There is a moratorium on the death penalty in Russia, but not in the territory it occupies in eastern Ukraine.

‘Propaganda above law’

Mr Aslin’s MP, Robert Jenrick, called for the Russian Ambassador to the UK to be summoned to the Foreign Office to “account for this egregious breach” of international law.

Ukraine’s Foreign Ministry told the BBC that all foreign nationals fighting for Ukraine’s military who are captured have rights as prisoners of war under international humanitarian law, and Russia was “forbidden to abuse, intimidate them or behave inhumanely with them”.

A spokesman described the “so-called trial” of all three men as “miserable”, adding the government would “continue to make every effort to release all defenders of Ukraine”.

“Such public trails place the interests of propaganda above law and morality, and undermine the exchange mechanisms of war prisoners,” he said.

Chairman of the foreign affairs committee Tom Tugendhat accused Russian President Vladimir Putin of carrying out “a form of hostage taking, a form of revenge”.

The Conservative MP told BBC Radio 4’s PM programme: “It’s not a state, it’s not a court and the judges are merely people dressing up and pretending.

“The reality is this is an absolutely brutal thing to do to three completely innocent people.”

The self-proclaimed Donetsk People’s Republic, or DNR, was set up by pro-Russian separatists in 2014.

It is not clear whether any executions have taken place in the breakaway republic – or if there will actually be any attempt to execute the men, the BBC’s international correspondent Orla Guerin said.

She explained there was little doubt DNR officials take their orders directly from Mr Putin and that the sentences appeared to be a tactic by the Kremlin to pressure and embarrass the British government over its military support for Ukraine.

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Prior to Russia’s invasion of Ukraine on 24 February, Mr Putin announced that he was recognising the independence of Donetsk and Luhansk, two breakaway regions of eastern Ukraine, in a move condemned by Nato and Western countries.

A month into its invasion, Russia scaled back its ambitions to capture Kyiv, and instead moved its focus to Donbas.

In Severodonetsk, a focal point of the war in in recent weeks, Ukrainian troops are engaged in intense street fighting with Russia in the battle for control of the city.

A No 10 spokesperson said: “We are obviously deeply concerned by this. We have said continually that prisoners of war shouldn’t be exploited for political purposes. You will know that under the Geneva conventions 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 and secure the release of any British nationals who were serving in the Ukrainian armed forces and who are being held as prisoners of war.”

Ukraine has given three Russian soldiers prison sentences for war crimes tied to the Russian offensive that began on 24 February. Vadim Shishimarin, 21, was sentenced to life in prison on 23 May for killing a 62-year-old civilian in Ukraine’s north-eastern Sumy region early in the war. On 31 May, Alexander Bobikin and Alexander Ivanov were each sentenced to more than 11 years in prison for shelling attacks on population centres that “violated the laws and customs of war”.

In a statement earlier this week, Aslin’s family said he had served in the Ukrainian marines for nearly four years and “is not, contrary to the Kremlin’s propaganda, a volunteer, a mercenary, or a spy”.

The family also accused Russia of violating the Geneva conventions by releasing video of Aslin “speaking under duress and having clearly suffered physical injuries”.

Robert Jenrick, Tory MP for Newark, said: “This disgusting Soviet-era show trial is the latest reminder of the depravity of Putin’s regime.

“Contrary to the Kremlin’s propaganda, Aiden Aslin and Shaun Pinner are not mercenaries. They have been living in Ukraine and serving in its armed forces long before Russia’s illegal invasion, and as prisoners of war they and are entitled to protection under the Geneva convention[s].

“The Russian ambassador should be summoned to the Foreign Office to account for this egregious breach of the Geneva convention[s]. No one should think they can treat British citizens like this and get away with it.”

“This so-called trial always had the appearance of a show trial designed to exert pressure on the UK, and these sentences look like they’re intended to fire a warning shot to the UK over its support for Ukraine in this brutal war,” he said.

“Russia and its proxies in the Donetsk People’s Republic will be adding to a massive catalogue of war crimes if they attempt to carry out these sentences.

“The UK and the UN and other bodies should inform Moscow that these sentences are completely unacceptable and must be quashed immediately