Claiming to be the most comprehensive assessment of the scientific literature on systemic pesticides to date, the IUCN's report says trace levels of pesticides can build up to cause long-term harm not only to invertebrates but also to grain-feeders such as birds.
It recommends a complete global phase-out of, or at least a significant reduction in, neonicotinoid use.
Pesticide Action Network UK policy officer Nick Mole said: "This important report makes it clear that those that have been concerned about the effects of neonicotinoids on pollinators have been right to be concerned."
But Crop Protection Association chief executive Nick von Westenholz described the report as "a selective review of existing studies which highlighted worst-case scenarios, largely produced under laboratory conditions".
He added: "As such, the publication does not represent a robust assessment of the safety of systemic pesticides under realistic conditions of use."
The Bee Coalition of environmental NGOs has said it will urge the government to take account of the IUCN's conclusions in the upcoming National Pollinator Strategy for England, expected in autumn 2014.