It follows a report in the Sun of a 19-year-old from the Coldstream Guards who left his Windsor barracks and bought a ticket to Poland at the weekend.
The Ministry of Defence did not confirm the report.
A spokesperson said all service personnel are banned from travelling to Ukraine until further notice.
“This applies whether the service person is on leave or not,” they added. “Personnel travelling to Ukraine will face disciplinary and administrative consequences.”
Senior UK military officers have been worried that some British troops – regulars or reservists – might try to join the battle in Ukraine.
The head of the British armed forces said at the weekend that Britons should not head to Ukraine to fight – and should instead help in sensible ways they can from the UK.
Adm Sir Tony Radakin said the “sound of gunfire” was not “something you want to rush to”.
And last week the Chief of Defence People, Lt Gen James Swift, sent out a message to the chain of command stressing that UK military personnel were “not authorised” to travel there.
He said that if there was any suspicion that troops were trying to make their way to Ukraine then it should be reported immediately to the service police.
The message warned that if serving British military personnel went to fight in Ukraine then they were putting not only their lives in danger but they also risked giving “the mistaken perception” to Russia that Britain had sent in troops to engage in hostilities.
The MoD advises against all travel to Ukraine, and says travelling to help in the fighting may be against the law or could lead to prosecution. The Ukrainian people can be supported in many ways, including through charitable donations, it stresses.
Previously, Foreign Secretary Liz Truss had said she backed individuals from the UK who might want to go to Ukraine to help the fight. Her comments came after Ukraine called on foreign nationals to come over and help.
The UK has repeatedly said it would not be sending British troops to fight in Ukraine, and the West has been clear that it would not engage in direct military confrontation.