Scott Sibley, who previously served with the British armed forces, was killed in the war-torn country on Saturday, while a second British man is said to be missing.
The families of both British nationals are being supported, a spokesperson at the Foreign Office said. No further details were given.
It has been reported the pair may have been volunteers fighting against Russian forces, but this has not been confirmed.
Friends of Mr Sibley, who leaves behind wife Victoria and a young family, paid tribute to him on social media and on a GoFundMe page set up in support of his family.
Gareth Stevens wrote: “Scott was a good man and I'm proud to call him a brother in arms.”
Craig Gant wrote: “Sadly on the 23rd April, 2022, Scott Sibley ‘Sibs’ was taken from us all. Scott was a son, father, brother and uncle. To me he was a friend like no other and the bravest person I've had the pleasure to have known, he was as good as a brother to me.
“His life may have been cut short, but he lived to the full and did things the Sibs way, right until the end.”
Alex Darwin said: “Sib, I'm in complete disbelief. What a pleasure to have served alongside you, to know you and to have experienced your kind nature.
'You were there for me and I will forever be grateful. One hell of a beautiful guy, inside and out.”
In 2018, Mr Sibley, from Northumberland, spoke to the media after shaving his head in support of his young daughter, Charlotte-Rose, who had been diagnosed with neuroblastoma.
A Devon-based Army squadron also paid tribute to Mr Sibley.
The Barnstaple-based Logistic Support Squadron said: “This week the Sqn has lost a former serving soldier. A man that showed Commando spirit until the end. RIP. Scott Sibley.”
In a statement the Foreign Office said: "We can confirm that a British national has been killed in Ukraine and are supporting their family."
On the missing person, the spokesperson said: "We are aware of a British national who is missing in Ukraine and are supporting their family. We are urgently seeking further information."
A small number of serving British personnel are believed to have gone absent without leave to join the resistance against the Russian invasion, while veterans and Britons without combat experience are thought to have also travelled to Ukraine.
There was initially confusion on the UK Government's position after foreign secretary Liz Truss, in comments during an interview to the BBC on February 26, said she would "absolutely" support UK nationals who chose to fight for Ukraine.
However, she later rowed back on those comments, insisting she had been "expressing support for the Ukrainian cause" in her remarks, and claiming there were "better ways" to contribute to the country's defence.
Veterans minister Leo Docherty has previously written to armed forces charities to encourage those tempted to travel to the war zone to turn their efforts to helping the Ukrainian people from the UK.
Russian Defence Ministry spokesman Igor Konashenkov warned in March that all foreign mercenaries detained in Ukraine would be treated as “mercenaries” and brought to justice on criminal charges.
It comes over a week after two British fighters – Shaun Pinner and Aiden Aslin – were captured by Russian forces.