Insect charity Buglife has said that records released by government research agency FERA show neonicotinoid seed treatments have been implicated in "One of the worst ever incidents of mass poisoning of wild bees in the UK."

The incident, in which almost 500 bees were found dead, occurred in Havering, Greater London. A resident of the area alerted local naturalist Tony Gunton after finding several dead bees next to a field of oilseed rape. The naturalist identified queen bees from three different species amongst the dead insects.

The oilseed rape was planted with imidacloprid seed treatments in autumn 2013, shortly before the EU's partial ban came into effect. The EU restrictions cover active ingredients imidacloprid, TMX and clothianidin.

Two types of fungicide were also detected, including flusilazole, which will be completely banned next month due to concerns over its environmental impact. Buglife claimed that evidence has suggested the 'cocktail' effects of the different chemicals may have amplified their impacts on the bees.

Buglife spokesperson Matt Shardlow said, "This is a timely reminder that despite the partial neonic ban there are perfectly legal flowering crops, treated with neonics last year, and are still harming our bees.  New research has revealed that our soils are heavily contaminated with these persistent pesticides and they are polluting rivers as well.  It is essential for the safety of our bees, pollinators and other wildlife that all neonic seed treatment use is suspended."

Industry groups, backed by the UK government, have opposed restrictions on neonicotinoid preparations. Though a growing body of evidence suggests the pesticides are harmful to wildlife at every level of the food chain, the government has called for more research into the compounds' environmental effects, while industry groups have said any action taken must be "evidence-based."

More information is available from Buglife