Russian servicemen patrol a territory of the seaport in the city of Mariupol, amid the ongoing Russian military action in Ukraine, on June 12, 2022. AFP
Jordan Gatley is the second Briton reported to have died fighting alongside Ukrainian forces against Russian invaders.
In a post on Facebook, his father Dean Gatley said his son had left the British Army in March and went to Ukraine "after careful consideration".
Gatley said his son was killed in the city of Severodonetsk, in eastern Ukraine, which has been under heavy Russian attack.
His Ukrainian comrades had praised "his wealth of knowledge, his skills as a soldier and his love of his job", the father said.
"He truly was a hero and will forever be in our hearts."
Asked about Gatley's death, a foreign ministry spokesperson said: "We are supporting the family of a British man who has died in Ukraine."
After fierce criticism from the UK military, Foreign Secretary Liz Truss was forced to backtrack after initially giving her blessing for Britons to go and fight in Ukraine.
In April, the government said one UK citizen had been killed in Ukraine and another was missing, with both reportedly involved in fighting against Russian forces.
The dead man was named by British media as military veteran Scott Sibley.
Another two Britons are facing the death penalty after they were captured and convicted as mercenaries by a pro-Russian court in eastern Ukraine.