Defence chiefs are warning military personnel against travelling to Ukraine to fight in the war amid reports several soldiers have gone AWOL to take on Vladimir Putin’s troops.

       Reports have suggested at least four British servicemen have left their duties to join Volodymyr Zelensky’s forces on the front line.

       Among them is a teenage Coldstream Guardsman, who abandoned his post protecting the Queen to travel to Ukraine, according to The Sun.

       The 19 year old left a note for his parents after booking a one-way ticket to Poland, the paper said, adding he planned to then cross the border to link up with Kyiv’s armed forces.

       The BBC said a defence source had confirmed the soldier was absent without leave. However, the Ministry of Defence would not confirm the reports, saying that it does not comment on individual cases.

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       A spokesperson said all service personnel are prohibited from travelling to Ukraine until further notice. “This applies whether the service person is on leave or not,” the spokesperson added, before warning that anyone service person who denies the order will “face disciplinary and administrative consequences.”

       All travel to Ukraine is banned under official UK military advice. Military chiefs are said to be concerned that, if captured, the soldiers could be used as an excuse by the Kremlin to accuse Britain of entering the war.

       The advice states that going to Ukraine to fight, or to assist others engaged in the conflict, may be against the law and could lead to prosecution.

       The UK also has limited consular support in Ukraine and is unlikely to be able to offer assistance to anyone in the country.

       This map shows the extent of Russia’s invasion of Ukraine

       (Press Association Images)

       It is understood that UK defence officials are keenly aware of the desire among Britons to help Ukraine after the Russian invasion, but the only support they can provide is defensive in nature.

       According to The Sun, the teenage Coldstream Guardsman is among as many as four missing British soldiers who are feared to have travelled to Ukraine to fight.

       The UK said it is working with its allies to provide a range of support to Ukraine, including to enhance that nation’s defence capability.

       Ben Wallace, the defence secretary who served in the Scots Guards, has previously urged Britons not to travel to Ukraine to join the fighting as he said the “very dangerous” situation could lead to them being killed.

       Mr Wallace has said he does not “want to see British people killed any more than I want to see Ukrainians” dying after Liz Truss, the foreign secretary, earlier said she would “absolutely” support British nationals who chose to go to help fight against the Russian invasion.

       Ukrainian armed forces simulate a crisis situation in an urban settlement in the abandoned city of Pripyat near the Chernobyl plant in the weeks before the Russian invasion

       (AP)

       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.

       The spokesman added: “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.”

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       Two weeks of war in Ukraine were marked on Wednesday morning.

       Both Ukraine and Britain’s Ministry of Defence said the Kremlin’s assault on capital Kyiv has for days failed to make any major progress and its advances across the rest of the country are slowing. Russia, meanwhile, insists the war is going as planned.