British troops who go AWOL to fight in Ukraine could face jail on their return - while the consequences of being captured by Russian forces would be dire, a legal expert warned.
The Army and the Ministry of Defence have said they are aware of a "small number" of soldiers who are believed to have travelled to fight.
But as Britain is not at war with Russia they could be wading into a legal quagmire, Dr David Turns, an international law expert from the University of Cranfield, told The Mirror.
And theoretically an 1870 law that hasn't been used for more than 120 years could punish civilians who make the journey to fight, he said.
Unless foreign fighters are integrated into the Ukrainian armed forces they have little protection under international law if they are caught by Vladimir Putin's army.
This means they could face prosecution as saboteurs, or a heightened risk of being shot, Dr Turns warned.
Meanwhile the government could enforce a law dating back to 1870 against civilians who go to fight, he said - although it was last used more than 120 years ago.
Dr Turns said: "The Armed Forces Act of 2006 covers going AWOL, that's a criminal offence under the act and it carries a maximum punishment of two years in prison.
"They could be liable for prosecution either for absence without leave or desertion, although I think that's less likely in these circumstances."
If troops were to go to Ukraine while on leave they could still face action under the Foreign Enlistment Act of 1870 - the same legislation that could theoretically be used against civilians, he said.
"It's never been repealed, so it's still on the statute books," Dr Turns said. "But it hasn't been used successfully since 1896 as far as I'm aware. It makes it a criminal offence for British citizens to take part in any hostility or any fighting against a friendly country.
"A friendly country in legal terms is a country that wasn't at war with the UK, and the fact is that the UK isn't at war with Russia.
"I'm aware from statements from people with legal backgrounds who have said there's a real risk that people could be prosecuted. That would be a decision for the legal officers in the government."
Brits who fight but are not formally inducted into the Ukrainian armed forces would not have the same protection under international law if they are captured.
Although the death penalty for prisoners of war is illegal, Dr Turns admitted that foreign combatants could face heightened risk, and the UK government would not be able to provide much help.
He said: "If they were captured they could run the risk of being prosecuted and that could open the possibility that they could be shot as saboteurs.
"The big risk is that they wouldn't be treated as members of the armed forces, they could be put up against a wall and shot or they could end up in front of a Russian court.
"They're pretty much at the mercy of the authorities that capture them, they could be put on trial as war criminals."
Ukrainian Foreign Minister, Dmytro Kuleba has previously said more than 20,000 volunteers from over 50 countries have joined up to fight.
On Wednesday a British Army spokesperson said: “We are aware of a small number of individual soldiers who have disobeyed orders and gone absent without leave, and may have travelled to Ukraine in a personal capacity. We are actively and strongly encouraging them to return to the UK.”
The MOD said in a statement: “All Service Personnel are prohibited from travelling to Ukraine until further notice.
"This applies whether the Service Person is on leave or not. Personnel travelling to Ukraine will face disciplinary and administrative consequences.”
