Britons Aiden Aslin and Shaun Pinner are accused of being mercenaries, Russian state media says, and have been sentenced to death.
They were captured in Mariupol in April during the intense fight for control of the port city, before appearing in court in the separatist Donetsk People's Republic (DPR).
It is understood they have admitted "training in order to carry out terrorist activities".
A third man, Moroccan national Saaudun Brahim, has also reportedly been sentenced to death.
The punishments were handed down by the DPR's supreme court, RIA said.
The men were found 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.
What were they doing in Ukraine?
Aiden Aslin has a Ukrainian fiancée, is applying for citizenship and joined the country's marines in 2018. Shaun Pinner has lived there since 2018, is married to a Ukrainian and was on his fourth tour of duty in the Ukrainian military.
So they were regular soldiers?
Yes. But Russia says they were mercenaries. It is making a distinction between them and other Ukrainian prisoners of war (POW) simply because of their place of birth.
Is that legal under international law?
No. Under the Geneva Convention, prisoners of war must be given certain protections. The British government says they are "entitled to combatant immunity and should be prosecuted for participation in hostilities".
So what is the UK going to do about it?
Downing Street said in a statement it would "continue to work with Ukrainian authorities to try and secure the release of any British national who were serving in the Ukrainian Armed Forces".
Ukrainian authorities obviously have no control over POWs being held by the Russians, but they are in negotiations with them over possible prisoner swaps.
So these two British men could be swapped for Russians?
It is possible. They appeared on Russian state TV appealing to be exchanged for pro-Russia Ukrainian oligarch Viktor Medvedchuk, who was arrested in April.
At the same time Medvedchuk appeared in a video released by the Ukrainians asking for the same thing.
So today's sentencing could be part of those negotiations?
It's possible. Medvedchuk is a close ally and friend of Vladimir Putin's. But Russia is also making a point about Ukraine's war crimes trials.
Once Kyiv started putting prisoners on trial it was only a matter of time before Russia did the same.
How likely is it they will be executed?
They have been told they have a month in which to appeal. If that fails and if they are not exchanged for prisoners, Russia warns they could face a firing squad.
Didn't Liz Truss encourage British people to go and fight in Ukraine?
She said she would absolutely support people going to fight there. Although it must be said these two men started fighting there long before that.
