Approximately £7 billion of military aid has been sent to the country to help fight off the Russian invasion, including hi-tech British anti-tank rockets.
Higher-grade systems are tracked but other equipment is now prompting fears that criminal gangs or terrorists could steal them to sell on the black market.
A security source told the Daily Mail that Eastern European gang members are entering Ukraine from Poland, paying cash for weapons and then returning to the EU.
EU law enforcement agency, Europol, has called for more checks to ensure shipments end up with the Ukraine military, while warning that there are no longer records from some weapons.
A spokesman said: “The war has caused a proliferation of firearms and explosives. Initially the Ukrainians kept records of weapons, but this was abandoned as the war progressed. Now there are no records.”
As well as weapons night vision goggles and body armour are also being bought.
Russia has posted videos on social media claiming to show Syria-based fighters with Western anti-tank weapons.
Clips uploaded anonymously to pro-Russian accounts apparently show anti-tank missile casings during black market sales in Ukraine.
However UK security officials believe the posts lack credibility as no geolocation tags or time-stamps are attached to the film.
A security source said: “It is easy to pose with the cases which missiles come in. It looks impressive but they’d need to steal the command launch unit and codes.”
Ukrainian government adviser Yuriy Sak said Kyiv was working hard to allay Western fears, with all movements of weaponry ‘closely monitored by ourselves and our international partners’.
The UK’s Ministry of Defence said it has protections in place.
A spokesman said: “We conduct a rigorous assessment of the risks and ensure an end-user agreement is signed by the armed forces of Ukraine.”