A British national has been killed in Ukraine and a second is missing.
The families of the two British nationals are being supported, a Foreign Office spokesperson said.
No other details were given.
Sky News reported that the pair may have been volunteers fighting Russian forces, but this has not been confirmed.
In a statement, the Foreign Office said: “We can confirm that a British national was killed in Ukraine and supports his family.”
On the missing person, the spokesperson said: ‘We know that a British national is missing in Ukraine and are supporting his family. We are urgently looking for additional information.
A small number of serving British servicemen are thought to have gone without leave to join the resistance against the Russian invasion, while veterans and Britons without combat experience also reportedly went to Ukraine.
There was first confusion over the government’s position after Foreign Secretary Liz Truss, in comments during a BBC interview on February 26, said she would ‘absolutely’ support British nationals who have chosen to fight for Ukraine.
However, she later backtracked on those comments, insisting that she had “expressed her support for the Ukrainian cause” in her remarks, and that there are “better ways” to help defend the country.
Veterans Affairs Minister Leo Docherty has previously written to Armed Forces charities encouraging those tempted to travel to the war zone to focus their efforts on helping the Ukrainian people in the UK.
Meanwhile, the latest government figures show that around a fifth of Ukrainian refugees who have been granted visas under the Homes for Ukraine scheme have arrived in the UK.
A total of 86,100 visas were issued on Wednesday under the Ukraine Family program and the Homes for Ukraine Sponsorship program, the government said.
But as of Monday, only 27,100 Ukrainians had arrived in the UK, according to figures from the Department for Levelling, Housing and Communities and the Home Office.
Overall, less than a third (31.4%) of those granted visas under the two schemes arrived in the UK.
There are many cases where families cannot travel to the UK because not all family members have received travel clearance, letters or visas.
There have also been significant concerns about the length of time it has taken for visas to be issued to refugees under the Homes for Ukraine program.
A government spokeswoman said: “We are processing thousands of visas a day – this shows that the changes we have made to streamline the service are working and we will continue to build on this success to speed up the process even further. “
Defense Secretary Ben Wallace said Britain and its allies must help drive Russian forces out of Ukraine like “a limpet from the rock”.
Mr Wallace said the UK would continue to supply arms to the Kyiv government and was examining options for supplying anti-ship missiles.
The British Ministry of Defense said that despite the losses of the cruiser Moskva and the landing ship Saratov, the Russian Black Sea Fleet retained the ability to strike targets in Ukraine.
In his latest intelligence assessment, he said the Russian Navy had about 20 ships in the area, including submarines.
Mr Wallace said it was vital that Ukraine, as a major grain producer, continued to have access to the Black Sea as an outlet for its exports.
“We have said that we will procure and supply, if we can, anti-ship missiles,” he told Sky News.
“It is extremely important that the grains that affect us all, food prices, get out of Ukraine, that the Russians cannot control the Black Sea.”
In a keynote speech on Wednesday, Ms Truss said the goal should be to drive Russia out of “the whole of Ukraine”.
Mr Wallace said the position of the international community had always been that Russia should withdraw from Ukrainian territory since the annexation of Crimea by President Vladimir Putin.
However, he said there was “a long way to go” before Ukrainians could reclaim land lost in 2014.
He said the Russians appeared to be preparing to dig into the territory they seized in the east of the country during their last invasion.
“We constantly said that Russia should leave the sovereign territory of Ukraine, so that has not changed,” he said.
“I think it’s certainly true that Putin, having failed in almost all of his goals, may seek to shore up what he has, to kind of fortify and dig in, like he did in 2014, and to to be just some kind of cancerous growth in the country of Ukraine and that it is very difficult for people to move them out of these fortified positions,” he said.
“So I think it’s really about, if we’re not going to have that happen, we need to help the Ukrainians get the limpet off the rock effectively and keep the momentum pushing them back.”