In a heavily edited video being circulated on encrypted messaging service Telegram, a man being interviewed identifies himself as Hayden William Davies, 30, and says he was born in Southampton.
It is not clear what conditions this video was filmed in, with it having been recorded after he was detained.
The bearded man, wearing military fatigues, says he was born on December 9, 1994.
The video jump cuts throughout as the man recalls his experience in the British Army and then speaks negatively about his decision to fight for Ukraine.
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A spokesperson from the Foreign, Commonwealth and Development Office told the Echo: “We are supporting the family of a British national who has reportedly been detained by Russia.”
The FCDO’s web page on Ukraine travel advice says: “If you travel to Ukraine to fight, or to assist others engaged in the war, your activities may amount to offences under UK legislation. You could be prosecuted on your return to the UK.
“The British Government’s ability to support you in these circumstances is very limited.”
The video comes after a similar post circulated online in November, where a man dressed in military clothing identified himself as James Scott Rhys Anderson from Banbury, Oxfordshire.
In his video, he said he formerly served in the British Army.
A Russian court confirmed on November 26 that he had been captured in the Kursk region.
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Today, Ukraine has halted Russian gas supplies to European customers that pass through the country after a pre-war transit deal expired.
Ukraine’s energy minister, Herman Halushchenko, confirmed on Wednesday morning that Kyiv had stopped the gas flows “in the interest of national security”.
He said on the Telegram messaging app: “This is a historic event. Russia is losing markets and will incur financial losses. Europe has already decided to phase out Russian gas, and [this] aligns with what Ukraine has done today.”
Russia launched an invasion of Ukraine almost three years ago on February 24, 2022.