LONDON—The illegal trade in counterfeit pesticides has grown into a multimillion-euro industry in Europe, putting consumers' lives and farmers' livelihoods at risk as unregulated and often toxic chemicals enter the food chain.

These untested and frequently substandard products can be hazardous to anyone handling them or to consumers buying contaminated food. Last year, for example, 28 metric tons of counterfeit pesticides destined for Lithuania were seized in Hamburg. Although they were packed alongside labels and measuring...