The "Watch Out!" campaign warns that such products are untested and uncontrolled, and may contain inferior or even dangerous and banned substances.
As well as endangering both crops and sprayer operators, their use can deprive farmers of their single farm payment and farm-assured status, create risks to wildlife, soil and water and even lead to prosecution.
Funded by the Crop Protection Association, NFU and Agricultural Industries Confederation, the year-long campaign is also supported by the Voluntary Initiative and Red Tractor Assurance.
They pointed out that the trade in illegal pesticides, often involving criminal gangs, is increasing.
Voluntary Initiative chairman Richard Butler said: "It is really sad that farmers now have to be aware of the danger posed by unscrupulous professional counterfeiters who want to undermine the professionalism and competence of UK farmers and sprayer operators."