"It is illegal for a British service person to... go absent without leave in the first place. But to go absent without leave in order to fight in a foreign war is simply unacceptable and frankly, risks the United Kingdom being wrongly claimed by Russia to be a belligerent in this," James Heappey told Sky News.

A handful of soldiers are believed to have gone absent without leave to join in the conflict and Heappey -- a former army officer who served in Afghanistan and Iraq -- warned they were now in "an awful lot of trouble".

"Service people who might think that they are doing the right thing should reflect that it is 100 percent not the correct thing to be doing," he added.

"There's a thing deep inside you as a soldier that makes you see TV images... and makes you want to go and fight to put things right but no good comes from British service people and veterans going to Ukraine to be a part of this."

He also cautioned those thinking of going to fight that they would be there for the long haul.

"People who think they can go there for a couple of weeks, take some selfies, get some Instagram shots and then come home.

"That is not the way the Ukrainians are viewing the people that go to fight for them," he added.

Foreign Secretary Liz Truss raised eyebrows last month when she said British people should join the fight if they wanted, a position that was later contradicted by government advice warning against all travel to Ukraine and from defence chiefs.

On Wednesday it emerged that a group of British former servicemen, including the son of a Conservative lawmaker, have travelled to Ukraine bent on fighting Russians.