The Department for Environment, Food and Rural Affairs is being investigated by the environmental watchdog after Conservative ministers authorised a bee-killing pesticide that was banned by the EU. The investigation into Defra was launched after the campaign group ClientEarth submitted a complaint to the Office for Environmental Protection, which was set up after Brexit to replace the EU’s framework for punishing environmental offences by governments in the bloc. On Monday, the OEP announced it would be investigating the emergency authorisation of a neonicotinoid pesticide in 2023 and 2024. It said: “The investigation is seeking to determine whether there were serious failures to comply with a number of environmental laws in relation to emergency authorisations granted for the use of Cruiser SB on sugar beet seeds. “In particular, the investigation is considering Defra’s interpretation and application of the precautionary principle and compliance with its nature conservation obligations when it considers granting emergency authorisations.” The neonicotinoid pesticide Cruiser SB is used on sugar beet and is highly toxic to bees and has the potential to kill off populations of the insect. It is banned in the EU but the UK has provisionally agreed to its emergency use every year since leaving the bloc. The former environment secretary Michael Gove promised in 2017 that ministers would use Brexit to stop the use of the pesticide. Instead, the EU banned all emergency authorisations of neonicotinoid pesticides while the UK government has allowed its use, one of many ways the UK has diverged from EU environmental policy since Brexit. Prof Dave Goulson, a bee expert at the University of Sussex, has warned that one teaspoon of the chemical is enough to kill 1.25bn honeybees. Conservative ministers authorised the pesticide for use this year, against the warnings of scientific advisers. Both the Health and Safety Executive (HSE) and the Expert Committee on Pesticides raised concerns over this year’s emergency authorisation. Neonicotinoid pesticides can stay in the soil for years, and they taint any flowering plant which grows, meaning that bees foraging for nectar from a flower can be poisoned years after treated seeds were planted. During the general election, the Labour party committed in its manifesto to end the authorisations of the pesticide because of its effect on bees. Kyle Lischak, of ClientEarth, said: “Failing to take a proper precautionary approach when approving the use of pesticides is threatening our pollinators and the wider environment – and puts England even further off-track from meeting its 2030 biodiversity targets. “This investigation also sends an important signal to government and other decision-makers: laws that protect nature – such as the habitat regulations – have to go beyond planning decisions and include other activities that could threaten nature. “And while this investigation is under way, we are calling on the new government to support UK farmers to adopt sustainable methods of pest control that work with nature, rather than putting it at risk – as they are the custodians of so much of England’s natural environment.” Richard Benwell, the chief executive of Wildlife and Countryside Link said: “This case will rightly investigate whether proper process was followed in allowing the use of banned pesticides, but if it is successful then the lessons are much wider. Scientific advice on major environmental decisions shouldn’t be swept under the carpet for political or economic expediency. We welcome Labour’s commitment to end the use of emergency authorisations for neonicotinoids, and hope the party will pay heed to the opinions of its expert advisers and the urgent need for ecological action in all its decisions, from toxic chemical use, to planning on land and at sea”. A Defra spokesperson said: “We are at a crisis point – nature is dying across Britain. This government will change existing policies to ban the use of bee-killing pesticides to protect our vital pollinators.”