British soldiers travelling to Ukraine to fight Vladimir Putin’s invasion risk creating a “dangerous situation”, a Cabinet minister has warned.

Grant 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 its 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.

The 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.

Ministry of Defence chiefs have banned all service personnel from travelling to Ukraine “until further notice”, while breaching the advice could lead to prosecution.

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.”

Asked what Russia might do if a British soldier was captured or killed, Mr Shapps said: “There’s a big difference between Britain sending its army in and some people who are breaking with our law and going to do it.

“But, clearly, this is a dangerous situation. And clearly, we want to make sure that the assistance we’re providing is done in an official way – like the anti-tank missiles that we provided prior to and during this conflict, and like the 22,000 Ukrainians that we’ve trained.”

The Coldstream Guards are the British Army’s longest continuously serving regiment and fought against the Russians in the Crimean War in the 19th century.

A Ministry of Defence spokesman added: “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.”

Defence Secretary Ben Wallace, who served in the Scots Guards, has previously urged British civilians not to join the fighting.

However, Foreign Secretary Liz Truss had said she would “absolutely” support UK nationals who chose to go to join the war.

Downing Street later effectively contradicted Ms Truss, as Prime Minister Boris Johnson’s official spokesman said: “We think the best way we can help Ukraine right now is by ensuring Putin fails.

“There are a number of ways Brits can show their support for that, and the Ukrainian embassy in London is putting out information about how British people can support.

“We fully recognise the strength of feeling about British people wanting to support the Ukrainians following the Russian invasion. There’s advice up on travelling to Ukraine, we currently advise against travel to Ukraine.”