PARIS — An influential European scientific body said on Wednesday that a group of pesticides believed to contribute to mass deaths of honeybees is probably more damaging to ecosystems than previously thought and questioned whether the substances had a place in sustainable agriculture.

The finding could have repercussions on both sides of the Atlantic for the companies that produce the chemicals, which are known as neonicotinoids because of their chemical similarity to nicotine. Global sales of the chemicals reach into the billions of dollars.

The European Commission in 2013 banned the use of three neonicotinoids — clothianidin, imidacloprid and thiamethoxam — on flowering plants after a separate body, the European Food Safety Authority, found that exposure to the chemicals created “high acute risks” to bees.

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