British soldiers who travel to Ukraine to fight can expect to be court-martialled, Boris Johnson has said, adding that civilians should also avoid going there to fight.

When asked about reports that a 19-year-old from Warrington with no military experience had travelled to the country to join efforts against the Russian invasion, the prime minister said that while he could comprehend why people wanted to help, they should remain in the UK.

“I think that everybody seeing what’s happening in Ukraine will understand those feelings,” he told reporters during a visit to the Cammell Laird shipyard in Merseyside.

“I think many people, many people in our armed forces, will sympathise because I don’t think I’ve ever seen such a 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.”

His comments are in notable contrast to those by Liz Truss, the foreign secretary, who said at the end of last month that she would back Britons going to Ukraine to take part in the fight against the Russians.

Truss faced criticism for her comments, which ran counter to advice on her department’s own website. This says that those who travel “to fight, or to assist others engaged in the conflict” could be prosecuted on their return to the UK.

A small number of serving British personnel are believed to have gone absent without leave to join the resistance, while veterans and Britons without combat experience have also travelled to Ukraine. Ministry of Defence officials have banned all service personnel from travelling to Ukraine “until further notice”, and warned they will face prosecution if they do.

Speaking earlier on Thursday, the 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.”

At the weekend the UK’s chief of the defence staff, Adm Sir Tony Radakin, said people should not “rush to … the sound of gunfire”, and ought to offer support in other ways.