Nearly a third of all weapons used by Isil on the battlefield were manufactured in the European Union, according to the most thorough investigation yet into how the jihadist group acquired its vast arsenal.

Islamic State of Iraq and the Levant (Isil) militants relied heavily on guns and ammunition produced by Romania, Hungary, Bulgaria and Germany, a report released on Thursday by Conflict Armament Research (CAR), an international organisation that documents weapons trafficking in war zones, revealed.

The only bigger producer of weapons used by the group was found to be China.

The revelation sits uncomfortably with the EU’s effort to degrade the group’s military capacity, CAR researchers say, and highlights how easily weapons can end up in the wrong hands in "messy" conflicts.

Their 200-page report provides the most comprehensive, verified study of the group’s weapons to date, presenting an analysis of more than 40,000 items recovered from Isil forces over three years.

It concludes that international weapon supplies intended for rebel factions in the Syrian conflict ended up with Isil, “significantly augmenting the quantity and quality of weapons in its arsenal”.