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The world’s most widely used insecticide has for the first time been officially labelled an “unacceptable” danger to bees feeding on flowering crops. Environmental campaigners say the conclusion, by Europe’s leading food safety authority, sounds the “death knell” for the insect nerve agent.

The chemical’s manufacturer, Bayer, claimed the report, released on Wednesday, did not alter existing risk assessments and warned against “over-interpretation of the precautionary principle”.

The report comes just months after the UK government dismissed a fast-growing body of evidence of harm to bees as insufficient to justify banning the chemicals.

Bees and other pollinators are critical to one-third of all food, but two major studies in March 2012, and others since, have implicated neonicotinoid pesticides in the decline in the insects, alongside habitat loss and disease. In April, the European commission demanded a re-examination of the risks posed by the chemicals, including Bayer’s widely used imidacloprid and two others.

Scientists at the European Food Safety Authority (EFSA), together with experts from across Europe, concluded on Wednesday that for imidacloprid “only uses on crops not attractive to honeybees were considered acceptable” because of exposure through nectar and pollen. Such crops include oil seed rape, corn and sunflowers. EFSA was asked to consider the acute and chronic effects on bee larvae, bee behaviour and the colony as a whole, and the risks posed by sub-lethal doses. But it found a widespread lack of information in many areas and had stated previously that current “simplistic” regulations contained “major weaknesses”.