University of Warwick's Professor of politics Wyn Grant said unregistered products marketed as soil "enhancers" or "strengtheners" were often being used as pesticides, which caused a "grey area" in the market.
Grant, who specialises in environmental policy, explained: "These products don't have to go through the registration process because they don't make specific claims about protecting from pests."
He told the biopesticide conference run last month by the Association of Applied Biologists: "We could get 'snake-oil' products, which damage the reputation of the industry."
Vivian Powell, a crop-protection manager at the Horticultural Development Company, said growers needed a clearer system of identifying which products were, or were not, registered and those that were exempt from registration.
"Some clear way of advising growers is needed so they don't make mistakes because they could get penalised through people supplying snake oils."
Consultant Rob Jacobson, secretary of the Cucumber Growers' Association, said this was a huge issue. Some of the products worked, others didn't. Some were "pure snake oil" and others' performance didn't match up to claims.
Sue Mattock, a pesticides expert at the Chemicals Regulation Directorate, said further guidance was "due shortly". This would focus on products containing active substances registered elsewhere in Europe, which would have to be reregistered.