Aiden Aslin, 28, was convicted of taking action towards violent seizure of power at a court in the self-proclaimed Donetsk People’s Republic.
Another Brit, 48-year-old Shaun Pinner, and Moroccan national Saaudun Brahim, were also convicted alongside Mr Aslin after the three were accused of being “mercenaries” fighting with Ukrainian troops.
Russian state media outlet RIA Novosti reported the three are set to face a firing squad.
Brennan Phillips, an American former soldier who met Mr Aslin in Syria and worked alongside him in Ukraine, said the judgement is a “provocation”.
Speaking to BBC Newsnight, Mr Phillips said: “I think it will invigorate people more than anything.
“Whatever effect they thought they would have in this provocation, I don’t think that and I don’t think it’s going to be well-received. And they did this as a provocation.”
Mr Phillips, from Tennessee, added that “many people expected” Russia to choose “the most provocative stance that they could take” – namely the death sentence.
“I do not believe that Sean or Aiden will be subject to the death sentence or anything like that,” he said.
“I do believe that their captivity under the Russians will be extended for a little bit, but I do believe wholeheartedly and I’m very confident that they will be released safely back to their families.”
The UK Government has insisted the sentences have no legitimacy and the pair should be treated as prisoners of war.
Russian news agency Interfax claimed the men would be able to appeal against their convictions.
Howard Morrison QC, the UK’s Independent war crimes Advisor to Ukraine, told the broadcaster the sentence has likely been employed for “negotiation purposes”.
He said: “I would hesitate to call it a judicial process, frankly.
“Judges have to be completely independent and have to act with an eye to due process and the requirements of international law and national law and the International Covenant on Civil and Political Rights, as well as the Geneva Conventions.
“I see precious little sign of any of that happening.”
Asked if the sentence is a negotiating ploy, Mr Morrison said: “”Well, it may well be. I mean, the trouble is, one is not sure.
“It’s very unlikely that it’s straightforward. There’s almost certainly going to be a subtext in there somewhere and the likelihood is that it is for negotiation purposes.
“In a sense, I hope so, because these men are under the death penalty, and the last thing you want is for that to be implemented.
“But if it is for negotiation, it’s difficult to see exactly what the subtext is – because the British government isn’t holding any prisoners, it’s the Ukrainians who are holding the prisoners.”
Mr Morrison said the Government’s complaint that the men are prisoners of war who should be treated according to the Geneva Conventions is “spot on”.
“There shouldn’t have been a show trial and there certainly shouldn’t have been a death penalty,” he said.
Mr Aslin, originally from Newark-on-Trent in Nottinghamshire, 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 conflict since Vladimir Putin’s invasion of Ukraine.
Mr Phillips said Mr Aslin had a well established life in Ukraine after arriving in 2017 and had not gone to fight there as a ‘thrill-seeker’.
He said: “He has a Ukrainian fiancee. They do have or did have a home outside of Mariupol and he was a part of the 36th Marine Brigade.
“So, yeah, he had a well-established life in Ukraine, Ukrainian citizenship.”