Sibley's death was confirmed today, 28 April, and another Brit is said to be missing, as the war in Ukraine continues.
The Foreign, Commonwealth and Development Office (FCDO) has not yet officially disclosed what the men were doing in the region.
However, the pair were said to be fighting as volunteers to support Ukraine's army, as reported by Sky News.
Speaking of Sibley, an FCDO spokesperson said: "We can confirm that a British national has been killed in Ukraine and are supporting their family."
Regarding the missing Brit, the spokesperson added: "We are aware of a British national who is missing in Ukraine and are supporting their family. We are urgently seeking further information."
There are currently no details about when or how Sibley was killed.
Ukraine is continuing to hold firm as an onslaught of attacks continues in the eastern Donbas region.
Just this week, on 26 April, Ukrainian forces were said to have destroyed a Russian command post on Snake Island.
According to a post from Ukraine’s Operation Command South, a Russian anti-aircraft missile system was also hit during the strike.
Snake Island has made headlines in recent months after a Ukrainian soldier who told Russian forces threatening to bomb the Ukrainian island in February to ‘go f**k themselves’ was awarded a medal for his services.
The man in question was Roman Hrybov, a border guard on the island, who was heard standing up to incoming Russian soldiers on a now-viral radio exchange.
As they approached, the Russian military warned: "This is a military warship. This is a Russian military warship.
"I suggest you lay down your weapons and surrender to avoid bloodshed and needless casualties. Otherwise, you will be bombed."
“Russian warship, go f**k yourself," Hrybov responded.
Hrybov would later receive the 'Merits To Cherkasy Region' award.
Upon receiving the honour the courageous man said: "I want to say a big thank you to the Ukrainian people for such support. We strongly feel this support, it inspires us."
If you would like to donate to the Red Cross Emergency Appeal, which will help provide food, medicines and basic medical supplies, shelter and water to those in Ukraine, click here for more informationFeatured Image Credit: Facebook