Footage on Russian television appeared to show Shaun Pinner, 48, in captivity, saying he was captured in Mariupol while fighting with the Ukrainian marines.
The former Royal Anglian soldier, who appeared tired in the video, said he had been fighting in the besieged city for five to six weeks but was now in the breakaway region of Donetsk.
He is the second British soldier fighting with the Ukrainian army to be paraded on Russian media after Aiden Aslin, 28, was filmed being led around in handcuffs with a cut on his forehead after surrendering to the Russian military in Mariupol last week.
In a statement released by the Foreign, Commonwealth & Development Office (FCDO), Mr Pinner’s family explained how he became involved in the defense of Ukraine, which they said he considers “his adopted country”.
The statement read: “Shaun was a well-respected soldier within the British Army serving in the Royal Anglian Regiment for many years. I have served on many tours including Northern Ireland and with the United Nations in Bosnia.
“In 2018 Shaun decided to relocate to Ukraine to use his previous experience and training within the Ukraine military.
“Shaun enjoyed the Ukrainian way of life and considered Ukraine as his adopted country over the last four years. During this time, he met his Ukrainian wife, who is very focused on the humanitarian needs of the country.
“I have progressed into the Ukrainian Marines as a proud member of his unit.”
The statement continued: “We would like to make it clear he is not a volunteer nor a mercenary, but officially serving with the Ukrainian Army in accordance with Ukrainian legislation.
“Our family is currently working with the Foreign Office along with the family of Aiden Aslin, who is also being held by the Russian Army to ensure their rights as prisoners of war are upheld according to the Geneva Convention.”
They described Mr Pinner as “funny, much-loved, well-intentioned” and said they hoped for a quick resolution to allow the captured men to return to their families.
“Our hearts go out to all those caught up in this horrific conflict,” the statement concluded.
Ukraine conflict: Ukraine vows to ‘fight absolutely to the end’ in Mariupol
The FCDO has been in contact with the families of Mr Pinner and Mr Aslin to support them.
However, the UK’s ability to obtain information and provide consular services on the ground is severely limited because of the conflict.
In a second heavily-edited video, Mr Pinner, who is originally from Bedfordshire, appeared to be questioned by a Russian journalist about how he was captured.
I answered: “We were in the factory area of Mariupol.
“In early hours of Tuesday morning, it was decided we move from the area of the factory, but we didn’t know exactly where.
“At about four in the morning we left the factory.”
There was “not much time to think,” he said.
The Russian reporter then appeared to tell Mr Pinner his Ukrainian commanders wanted him to be killed.
Mr Pinner spoke of his fear of capture in January, telling the Mail on Sunday: “I fear for my life. The Russians will treat us differently if we are captured because we are British. This is always on my mind, that I will be captured.”
Mr Aslin, originally from Nottinghamshire, had been defending Mariupol with his unit during heavy fighting in recent weeks before having to surrender after 48 days.
“We have no food and no ammunition,” a post on his Twitter account, which was being run by a friend while he was fighting with the Ukrainian marines, read.
Mr Aslin’s grandmother told the PA news agency that a video on Russian television showing him saying Ukraine was not making “the right decisions” was “propaganda”.