The organisations claim: "There is now more than enough evidence to retain the ban and extend it to all crops, and that this is essential to reverse the decline of bees and other pollinators."
Industry organisations such as the NFU and HTA have opposed the ban and called for more scientific research on neonicotinoid use.
NFU horticulture chief adviser Dr Chris Hartfield said: "The fact is the evidence is not clear on the issue of bees and neonicotinoids ‘there still remain major gaps in our understanding’ and ‘there is still a limited evidence base to guide policymakers on how pollinator populations will be affected by neonicotinoid use’.
"These aren’t my words. These are statements made in an independent study reviewing all the current evidence arounds neonicotinoids and pollinators. A review written by Professor Charles Godfray & Professor Angela McLean FRS (both from the Oxford Martin School at the University of Oxford), Dr Tjeerd Blacquière, Wageningen University; Professor Linda Field, Rothamsted Research; Professor Rosemary Hails & Dr Adam Vanbergen, Centre for Ecology & Hydrology; Professor Simon Potts, Reading University; and Professor Nigel Raine, Guelph University.
"These are serious heavyweight international experts in insecticides, ecology, beekeeping, toxicology, mathematical biology, biodiversity and pollinators. So when we say the evidence around neonicotinoids and bees is unclear – it is not me saying this, it is not the NFU saying this, it is what independent experts are saying.
"These experts published a review in 2014 and an update at the end of last year and the reviews conclude that there is limited evidence to guide policy makers*.
"So when you hear someone telling you that the evidence around neonicotinoids and bees is clear, and that it supports a ban on the use of all neonicotinoids, it rings alarm bells. What they are actually saying is that the evidence is clear enough for them. That is a very different thing from the evidence being clear for independent and impartial experts, and policymakers."
The EU restrictions, which ban the use of three neonicotinoids on flowering crops, is due to be reviewed in 2017, starting with a comprehensive assessment of the scientific evidence of the threat posed by the pesticides, by the European Food Safety Authority (EFSA).
The EU-wide moratorium, which came into force in December 2013, restricts the use of three neonicotinoids: imidacloprid, clothianidin and thiamethoxam, on flowering crops.
In the letter, the organisations - which include Friends of the Earth, RSPB, Greenpeace, The Wildlife Trusts, Buglife, Butterfly Conservation and Bat Conservation Trust - say: "Since 2013 many more independent laboratory and field studies have found neonics impairing the ability of different bee species to feed, navigate and reproduce resulting in declining populations.
"The government says it will not hesitate to act on evidence of harm. The third anniversary of the neonics restrictions is Environment Secretary Andrea Leadsom’s chance to catch up with scientific evidence and public opinion by keeping and extending the ban as part of properly protecting Britain’s bees and pollinating insects."
Dave Goulson, Professor of Biology at Sussex University said: "Three years ago EFSA's analysis of the scientific evidence concluded that neonicotinoids "pose an unacceptable risk to bees". Since then dozens of new studies from around the world have been published, including a major Swedish field trial in which neonicotinoids were shown to impact profoundly on bumblebee colonies and solitary bees.
"Work from Italy has showed that even tiny doses of neonicotinoids impair the immune system of honeybees, rendering them susceptible to infections. Perhaps more concerning, it has become clear that neonicotinoids are persistent and pervasive in the environment, so that soils, wildflowers, ponds and rivers commonly contain significant levels.
"This widespread pollution of the environment with these potent neurotoxins has now been linked not just to bee declines but also to declines in butterflies, aquatic insects, and insect-eating birds. With farmland wildlife populations in free fall, it is surely time to extend the moratorium on neonicotinoids to cover other uses."
Dr Penelope Whitehorn, Applied Ecologist, University of Stirling, said: "The scientific evidence now clearly shows that neonicotinoids are causing massive harm to bees and other species that we all depend upon. These chemicals should go the way of DDT and be permanently discontinued.
"What’s more, there is plenty of evidence that alternative pest control strategies really work. It is now vital that our government properly supports farmers to gain the knowledge and tools to maximise yields and minimise chemical inputs using Integrated Pest Management. This is the path to a more sustainable future".
Dr. Christopher Connolly, Reader in Neurobiology, University of Dundee, said: "The evidence from our research shows a complex relationship between different neonicotinoids and different insects. What is safe to one species may be very toxic to another.
"We can’t afford to take any risks with our insect pollinators so to protect them the moratorium must stay in place for the three restricted neonicotinoids.
"And we now know that these chemicals are so persistent that they can turn up, not just in the treated crop, but in the pollen or nectar of wildflowers or crops grown subsequently in the same field. To prevent bees being exposed in this way the restrictions need to be extended to other crops including wheat where neonics are still widely used".
The open letter to government:
"December 1st marks the third anniversary of the introduction of Europe-wide restrictions on three neonicotinoid pesticides - often known as 'neonics' - after they were found by scientists to pose a "high acute risk" to honeybees.
It is clear that there is now more than enough evidence to retain the ban and extend it to all crops, and that this is essential to reverse the decline of bees and other pollinators.
Since 2013 many more independent laboratory and field studies have found neonics impairing the ability of different bee species to feed, navigate and reproduce resulting in declining populations.
There is now solid evidence of harm from neonics to wild bumble and solitary bees which are even more sensitive to these pesticides than honeybees. Evidence has also grown of neonics harming the wider environment with studies indicating a link to butterfly population decline, identifying risks to bird species and finding neonics accumulating to dangerous levels in wildflowers surrounding crops.
2017 will be a crucial year for decisions on bees as scientists will publish the official review of the evidence of harm to bees from the three restricted neonicotinoids.
The government says it will not hesitate to act on evidence of harm. The third anniversary of the neonics restrictions is Environment Secretary Andrea Leadsom’s chance to catch up with scientific evidence and public opinion by keeping and extending the ban as part of properly protecting Britain’s bees and pollinating insects.
Craig Bennett, Chief Executive, Friends of the Earth
Dr Jeremy Biggs, Director, Freshwater Habitats Trust
Kath Dalmeny, Chief Executive, Sustain
Julia Hanmer and Kit Stoner, Joint Chief Executives, Bat Conservation Trust
Martin Harper, Conservation Director, RSPB
Heidi Herrmann, Co-Founder, Natural Beekeeping Trust
Dr Maggie Keegan, Head of Policy, Scottish Wildlife Trust
Peter Melchett, Policy Director, Soil Association
John Sauven, Executive Director, Greenpeace UK
Matt Shardlow, Chief Executive, Buglife
Steve Trent, Executive Director, Environmental Justice Foundation
Steve Trotter, Director, The Wildlife Trusts
Dr Keith Tyrell, Director, Pesticides Action Network
Dr Martin Warren, Chief Executive, Butterfly Conservation
Catherine Weller, Head of Biodiversity Programme, ClientEarth
*A restatement of the natural science evidence base concerning neonicotinoid insecticides and insect pollinators. 2014. H. Charles J. Godfray, Tjeerd Blacquière, Linda M. Field, Rosemary S. Hails, Gillian Petrokofsky, Simon G. Potts, Nigel E. Raine, Adam J. Vanbergen, Angela R. McLean http://rspb.