Demands for a permanent ban on nicotine-based pesticides have been prompted by a major new study suggesting they can be harmful to bees.
The huge £2.7 million investigation, spanning the equivalent of 3,000 football pitches in three European countries, provided the first real-world evidence that vulnerable honeybee colonies suffer from exposure to the chemicals.
It also indicated that - like many of the British human population - honeybees in the UK are the victims of bad diet and ill-health.
Researchers found that the pesticides, known as neonicotinoids, had country-specific effects. While they reduced the survival of honeybees in the UK and Hungary, the chemicals caused no harm to the insects' counterparts in Germany.
There were even signs that use of the insecticides temporarily benefited German honeybee populations.
Professor Richard Pywell, a leading member of the UK team from the Centre for Ecology and Hydrology (CEH), said: "Variation among countries was found and this suggests that the effects of neonicotinoids are influenced by other factors, such as what the bees are feeding on in the landscape, and disease."
Currently neonicotinoids, coated on seeds to be incorporated into growing plants, are banned in Europe for mass-flowering crops because of previous research suggesting they might be harmful to pollinators.
A decision on whether or not to continue or extend the moratorium is expected this autumn.
Environment groups said Britain should back a permanent and all-encompassing neonicotinoid ban in light of the new findings, published in the journal Science.
Friends of the Earth nature campaigner Sandra Bell said: "This crucial study confirms that neonicotinoid pesticides come with a nasty sting in the tail for our under-pressure bees. It's time for a complete and permanent ban on these chemicals.
"The UK Government must stop asking for yet more evidence and back tough action on these dangerous chemicals to protect our precious pollinators."
Doug Parr, UK chief scientist at Greenpeace, said: "This major study marks a watershed moment in the fight to protect our bees.
"The case for a permanent ban on these pesticides is now unassailable, and our politicians will have to take action."
And Vito Buonsante, from environmental law firm ClientEarth, said: "This is yet more evidence that the EU should adopt a total ban on these chemicals."
The research showed that exposure to neonicotinoid-treated crops reduced the ability of British and Hungarian honeybees to sustain themselves over the winter and survive through the following spring.
Colony numbers fell by 24% in Hungary. In the UK, colony survival was already low even without the pesticides. But death rates were highest when the bees gathered nectar and pollen from oil seed rape treated with neonicotinoids.
Honeybees in Germany were generally unaffected by the chemicals. Although there was some evidence of the pesticides leading to greater reproductive success in German hives, this positive effect disappeared after three to six weeks.
The study also examined the fate of two wild bee species, the buff-tailed bumblebee (Bombus terrestris) and solitary Red Mason Bee (Osmia bicornis).
For both these species, poor reproductive success either due to lack of queens or low egg production was linked to increasing neonicotinoid residues in nests.
Diet and disease are thought to be two key factors influencing bees' response to the pesticides.
In the both the UK and Hungary, honeybees have a seriously restricted diet, largely due to the loss of wild flowers from farmland.
Oil seed rape accounts for 40% to 50% of the honeybee diet in both countries - but only 15% in Germany.
"It suggests that German honeybees have better diets, less reliant on oil seed rape," said Prof Pywell.
British honeybee hives also suffered higher levels of infection by the Varroa parasitic mite than their German counterparts. Hungary had the highest levels of infection by a different parasite, the Nosema fungus.
Dr Ben Woodcock, who led the CEH team, said: "There may be opportunities to mitigate negative impacts of neonicotinoid exposure on bees through improved honeybee husbandry or availability of flowering plants for bees to feed on across non-cropped areas of the farmed landscape."
The trial was mostly funded by two leading manufacturers of neonicotinoids, Bayer and Syngenta.
In separate statements, both companies pointed to the inconsistency of data from the three countries and drew attention to the "positive" effects seen in Germany.
To counter claims of potential bias, an independent committee of scientists was appointed to oversee the research.
A spokeswoman for the Environment Department (Defra) said the Government based all its decisions on pesticides on scientific evidence.
She added: "Bees and other pollinators are vital to the diversity of our environment and food production which is why we are leading a nationwide strategy to better protect them.
"We are encouraging farmers to provide the food and habitats pollinators need on their land, as well as promoting simple actions the public can take to help such as cutting grass less often and growing pollen-rich plants."