The Civil Guard began investigating after detecting urban waste that had entered Spain with forged documents and was destined for landfills without being treated as required by law.

They discovered several companies based in the regions of Catalonia and Castilla-La Mancha hired by Italian waste management firms to receive and dispose of the garbage.

This allowed the Italy-based firms to dodge taxes and save costs, breaking environmental rules that stipulate each country must deal with its own rubbish, the Civil Guard said in a statement.

The organisation earned more than 19 million euros ($19.5 million) and dumped more than 40,000 tonnes of waste in Spanish landfills per year since 2021, the Civil Guard estimated.

The landfills in Tarragona and Cuenca provinces received "all types of urban waste... including dangerous and toxic waste", potentially endangering local ecosystems and public health.

The European Union has prioritised fighting environmental crime networks, which according to the Civil Guard is the world's fourth-largest organised criminal activity after drug smuggling, human trafficking and forgery.