More than one in 20 pesticides sold in the EU could be fake, potentially endangering food safety and human health, according to the Society of Chemical Industry.
These counterfeits range from sophisticated copies of patented products to low-quality fakes with little or no resemblance to the original.
The European Crop Protection Association (ECPA) added that this is a growing problem.
"Clearly there are risks when products that have not been properly studied or evaluated are being brought onto the market," Roger Doig, president of the ECPA told the society's magazine Chemistry & Industry.
Several recent incidences highlight the extent of the problem. In February this year, a counterfeit herbicide used in Italy was found to contain quantities of a potentially dangerous insecticide. And a 2002 study of supermarket produce in the UK found traces of eight illegal and potentially dangerous compounds.
The EU crop protection market is worth 7.5bn, and counterfeits cost the EU between 21m and 30m a year in lost taxes. As a result, the ECPA recently launched a pan-European Anti-Counterfeit Programme, in an effort to get governments and regulators to use their powers to enforce regulatory policies.
In the UK for example, the Department for Environment, Food and Rural Affairs (DEFRA) is currently investigating two companies suspected of acting illegally. Jean Train, spokesperson for the Pesticides Safety Directorate told the magazine that the department was in the process of gathering evidence "with intention to prosecute".
This will add to the list of successful action PSD have already taken against companies dealing in illegal pesticides. Twenty-four companies were issued with warnings in October at the British Crop Protection Conference in Glasgow for illegally promoting products.
But Peter Sanguinetti, CEO of the UK Crop Protection Association (CPA) argued that the UK is in fact ahead of the game. Counterfeit products account for three per cent of the UK market, compared to five to seven per cent in the EU.
"The CPA actively encourages enforcements to prevent illegal imports," he said. "We recommend that farmers buy pesticides from a reliable source. CPA members sign a code of practice."
Consumer awareness of pesticides has grown in the last few years, and is one of the drivers behind the current boom in organic food. In addition to concerns about counterfeit products, pesticide residues in food found to be above maximum permitted levels have also hit the headlines.
Defra's Pesticide Residues Committee reported in September that seven samples out of 366, or 1.9 per cent, contained residues above the MRL. Illegal MRLs were found in one sample of grapes imported from outside the EU, four samples of lettuce, and two speciality fruits.
The MRL, or maximum residue level, is the maximum concentration of a pesticide residue - expressed as milligrams per kilogram, or parts per million - legally permitted in or on our food and animal feeds. The committee stated however that "none of these residues were likely to cause concern for people's health."
The committee has asked suppliers and the authorities in exporting countries for an explanation of the findings.