Images on Russian television appeared to show Aiden Aslin, 28, from Nottinghamshire, being led around in handcuffs with a cut on his forehead.
His mother Ang Wood, of Balderton, Nottinghamshire, told the Telegraph she believes the images were legitimate due to a tattoo on her son’s arm that said “Happy Days”.
She called on the Russian government to treat her son as a prisoner of war in accordance with international rules.
Ladies Day at Musselburgh Races: Children's nurse crowned most stylish woman
Edinburgh dad Joseph Wakeley dies days after being hit by car near Sheriffhall Roundabout
Livingston crash: 28-year-old woman pronounced dead at scene as man arrested in connection with incident
Pedestrian dies after being hit by car in Edinburgh
Edinburgh crime: councillor Kevin Lang left shaken after being ‘grabbed by throat’ while delivering newsletters
Ms Wood told the Telegraph she was “in bits”, adding: “I now hold Vladimir Putin to the terms of the Geneva Convention. Aiden is a serving member of the Ukrainian armed forces and as such is a prisoner of war and must be treated with humanity.
“It already looks like he has been beaten up. It is time now for the British Government to get involved and help secure Aiden’s release.”
It is understood the Foreign, Commonwealth and Development Office is still working to verify the reports of Mr Aslin’s capture and that its ability to provide consular services in Ukraine is severely limited due to the conflict.
But officials expect any detainees to be treated “in accordance with international law”.
The Telegraph reported Mr Aslin enlisted in the Ukrainian army in 2018 and that he had previously fought against the so-called Islamic State in Syria.
Footage broadcast in Russia suggested he had been captured following fighting in Mariupol.
Footage of Aiden has been broadcast across Russian TV including on Russia Today.