Fears over the effects on bees of neonicotinoid insecticides - among the most commonly used crop pesticides in the world - led France to withdraw approval in June last year for Syngenta's Cruiser OSR, used to treat rapeseed crops.
Responding to the opinion by the European Food Safety Authority (EFSA), the European Commission said it was ready to take the necessary steps if its findings are confirmed, raising the prospect of EU-wide restrictions on the use of the products.
As EurActiv reported in November, EFSA launched a review of scientific studies and risk assessments in response to the decline in bee populations worldwide and in Europe.
The three pesticides analysed by EFSA were clothianidin and imidacloprid, both of which are primarily produced by Bayer's agricultural unit Bayer CropScience, and Syngenta's thiamethoxam, the active ingredient in Cruiser OSR.
EFSA said harmful pesticide residues in the pollen and nectar of plants treated with the three chemicals meant that they should only be used on crops not attractive to honeybees, such as sugarbeet. That would exclude their use on maize, rapeseed and sunflower crops.
Bees also face an acute risk from exposure to drifting pesticide dust following sowing of cereal seeds treated with the chemicals, including wheat and barley, EFSA said.
However, the EU scientists found no link between use of the pesticides and the phenomenon known as bee colony collapse disorder, which has seen bee populations fall rapidly in recent years across Europe and North America.
"Due to shortcomings in the data, EFSA was unable to finalise assessments for long-term risks to colony survival and development ... and therefore conclusions could not be drawn on colony collapse disorder," the opinion said.
Bayer CropScience said in a statement it did not believe EFSA's findings altered the conclusions of previous EU assessments of its products, which found no unacceptable risks in their use.
"It is very important that any political decision relating to registrations of neonicotinoid-containing products should be based on clear scientific evidence of adverse effects of the affected products under realistic conditions of use," the statement said.
A study by Britain's Food and Environment Research Agency said last year that there was no evidence that use of neonicotinoids could cause the collapse of whole bee colonies.
EFSA said current gaps in the scientific data meant it had been unable to assess the risks posed to bees by some of the authorized uses of the pesticides, and there was a high level of uncertainty in its latest evaluation.