A teenage Coldstream Guard has left his post guarding the Queen to join the fight in Ukraine, ITV News understands.
Army sources confirmed a report in The Sun that a 19-year-old guardsman had left his Windsor barracks to join the battle against Russian invading forces.
The newspaper reported that the teen had left a note for his parents and booked a one-way flight to a country in Eastern Europe, which was not identified.
On Wednesday, Foreign Secretary Liz Truss withdrew her support for British nationals to go to join the fight after being contradicted by colleagues.
She insisted that she had been just "expressing support for the Ukrainian cause" when she earlier said she would "absolutely" support UK nationals who chose to go to join the war.
Now Ms Truss said there are "better ways" to contribute to the efforts, such as making donations, as she stressed the travel advice for individuals not to go to Ukraine.
A statement from the 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."
The MoD added it would not comment on specific cases. All travel to Ukraine is banned under official UK military advice.
It 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.
Another issue is that the UK has limited consular support in Ukraine, and is unlikely to be able to offer assistance to anyone in the country.
It is understood that UK defence chiefs are keenly aware of the desire among people back home who want to help the Ukrainians after the Russian invasion, but the only support they can provide is defensive in nature.
The UK is working with its allies to provide a range of support to Ukraine, including to enhance that nation’s defence capability.
Defence Secretary Ben Wallace, 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 kill.