The grenades, dating from 1925, were found in a consignment of onions heading from France to a factory in the northern Dutch province of Zeeland.

Dutch authorities asked the OPCW to examine the find.

"The projectiles were identified to be consistent with pre-1925 French and German manufactured old chemical weapons," The Hague-based organisation concluded after peeling back the layers of the case.

"However, the type of chemical fill could not be identified," it added in a statement.

The reason for the weapons' presence in the delivery remains a mystery.

The grenades will be destroyed, but since the Netherlands lacks a dedicated destruction facility for this type of object, they will be "transferred as toxic waste to the specialised chemical dismantling facility in Poelkapelle, Belgium," the OPCW said.

The Netherlands thanked the Belgian authorities for their help.