A small number of British solders have “disobeyed orders and gone absent without leave” and may have travelled to Ukraine “in a personal capacity”, the British Army has said.
An Army spokesperson said: “We are strongly encouraging them to return to the UK.”
It comes after Defence Secretary Ben Wallace said soldiers could be prosecuted for desertion if they travelled to Ukraine to fight against Russian forces.
Ministry of Defence (MoD) chiefs have banned all service personnel 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,” a MoD spokesperson said.
Mr Wallace stressed that serving soldiers who have reportedly already travelled to the war zone will face a court martial when they return.
He also warned other Britons not to travel to Ukraine, telling them they would not be there for “a selfie and six weeks” but in the war “for real”.
In the Commons, Mr Wallace said any serving personnel who went to Ukraine “will be breaking the law and they will be prosecuted when they return for going absent without leave or deserting”.
For other Britons, he said “we strongly discourage them from joining these forces” in Ukraine.
“The Ukrainians are very clear: you turn up, you are in it for the whole game. You are not in it for a selfie and six weeks, you are in it for real,” he said,
“I think we have seen already some people at the borders find that may be not be the right option to follow.”
The UK’s position on Britons going to join the resistance to Vladimir Putin’s invasion has seen ministers forced to row back on comments from Foreign Secretary Liz Truss, who said she would “absolutely” support volunteers who went to Ukraine to fight.
Ministers’ concerns about Britons being caught up in the fighting were underlined by Cabinet minister Grant Shapps, who said it risked creating a “dangerous situation” with the Russians.
Mr Shapps told troops “you cannot just get up and go” after a 19-year-old Coldstream Guard was reportedly among up to four missing British soldiers feared to have travelled to fight Russia.
The Transport Secretary stressed there was a “big difference” between individuals taking the potentially illegal decision and the UK dispatching troops to fight alongside Ukrainians.
Ministers have ruled out such a move over fears it could spark a world war, instead committing to providing defensive weapons to fend off Moscow’s troops.
A teenage Coldstream Guardsman, based in Windsor barracks, wrote a goodbye letter to his parents and bought a ticket to Poland over the weekend with the aim of crossing into Ukraine, according to The Sun.
Mr Shapps told ITV’s Good Morning Britain: “You cannot go and fight if you’re in the British Army, you cannot just get up and go and fight.
“Of course that’s inappropriate behaviour and you would expect the army to have some very, very strict rules in place, as they do.”