Russia could use the presence of British troops who have broken the law to fight in Ukraine as an excuse to view the UK as a “belligerent” in the conflict, ministers fear.

A small number of serving British personnel are believed to have gone absent without leave to join the resistance against the Russian invasion, while veterans and Britons without combat experience have also travelled to Ukraine.

Boris Johnson warned they would face court martial on their return to the UK, as Ministry of Defence (MoD) chiefs have banned all service personnel from travelling to Ukraine “until further notice”.

Armed forces minister James Heappey said: “It is illegal for British service personnel to first of all 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.

“Service people who might think they are doing the right thing should reflect it is 100% not the correct thing to be doing.”

He said he had heard a small number of serving troops – around three or four – had gone to Ukraine.

But he added: “It’s not just the very small number of soldiers who have gone Awol apparently to go fight in Ukraine, I would say to the wider veterans community I know… you accept the unlimited liability in the service of your country to stand up for what’s right in the world, that there’s a thing deep inside you as a soldier that makes you see TV images and makes you want to go and fight for what’s right, but no good comes from British service people or veterans going to Ukraine to be a part of this.”

In recent days ministers and defence chiefs have scrambled to persuade Britons not to travel to join the fighting in Ukraine.

In a visit to Merseyside, the Prime Minister told broadcasters: “I think that everybody seeing what’s happening in Ukraine will understand those feelings and I think many people, many people in our armed forces, will sympathise because I don’t think I’ve ever seen such clear distinction in international affairs between right and wrong and good and evil in what President Putin is doing to people in Ukraine.

“But we have very clear laws in this country. You shouldn’t go to Ukraine, and I’m afraid people going from our armed services, as the Chief of the Defence Staff made clear the other day, will face court martial.”

Their interventions follow comments from Foreign Secretary Liz Truss on February 27 saying she would “absolutely” support Britons going to fight “if that’s what they want to do”.

Some veterans have reportedly received cold call approaches from private militia groups inviting them to join the fighting in Ukraine.

The BBC reported the militia groups had been scouring social media to recruit ex-soldiers.

Concern within ministerial ranks about the situation in Ukraine led Defence Secretary Ben Wallace to warn on Wednesday that people travelling to the conflict zone would not be there for “a selfie and six weeks” but would be in the war “for real”.