Most uses of pesticides known as neonicotinoids represent a risk to wild bees and honeybees, European experts have warned.
A new assessment confirming the dangers of three neonicotinoid pesticides on bees has prompted calls for a total ban on the use of the chemicals in the countryside.
Use of the three pesticides is already restricted in the European Union on crops such as oil seed rape, because of the concerns they have "sub-lethal" effects such as harming the bees' ability to forage and form colonies.
The assessment by the European Food Safety Authority (Efsa), which looked at the impacts on wild solitary bees and bumblebees as well as honeybees, has confirmed that most uses of the chemicals pose a risk to the insects.
It updates a 2013 analysis which led to the European Commission imposing the controls on use of the substances.
The European Commission has since proposed extending restrictions to only allow use on plants in greenhouses, extending the ban to crops such as sugar beet and winter cereals with seed treated with the chemicals.
Environment Secretary Michael Gove has said a total ban in the countryside would be supported by the UK and kept in place after Brexit.
Jose Tarazona, head of Efsa's pesticides unit, said that "overall the risk to the three types of bees we have assessed is confirmed".
Bees can be exposed to the pesticides by foraging on flowering crops in the field, from pollen and nectar which contain residues of the chemicals.
They can also be exposed through plants in the vicinity of the crop which have been contaminated by dust drifting away from the field and through contaminated soil, Efsa said.
Friends of the Earth called for an urgent outdoor ban on the chemicals, with campaigner Sandra Bell warning "we have been playing Russian roulette with the future of our bees for far too long".
"The UK Government has already said it will support a complete ban on the outdoor use of these three bee-harming chemicals - a move that is fully justified by this report. Other EU countries must now back a tougher ban too.
She added: "Ministers must also use their post-Brexit farming policy to help our farmers to work in harmony with nature - and not against it.
"This must include an overhaul of the pesticide approval process, and a reduction in their overall use."
Commenting on the report, Dr Philip Donkersley from Lancaster University, said: "All three neonicotinoids evaluated here show high risk for exposing bumblebees through the pollen and nectar that they feed on.
"This exposure affects solitary bee reproduction, colony viability and learning ability in bumblebees."
Prof Christopher Connolly, from the University of Dundee, said the study showed high risks did not come from direct contact to non-flowering crops, but from indirect exposure in field margins and nearby crops or those grown afterwards.
He warned the greatest risk to bees was from ongoing exposure due to the persistence of the pesticides and their transfer to other flowering plants, and constant low-level pollution could also increase the resistance of pests to the chemicals.
"A highly restricted use of neonicotinoids would reduce this environmental stress and retain neonicotinoids as important pest control agents for use in severe situations," he suggested.