The Government may have failed to comply with environmental laws when it granted use of a banned pesticide toxic to bees, a watchdog has warned.

The Office for Environmental Protection (OEP) is investigating the Environment Department (Defra)’s emergency authorisation for the use of the neonicotinoid thiamethoxam on sugar beet seeds in 2023 and 2024.

As part of the investigation, launched in June, the environmental watchdog has issued an “information notice” to Defra setting out alleged failures to comply with a number of environmental laws, why it is serious, and requesting specific information relating to the allegations.

The department has until January 28 2025 to formally respond to the notice.

The pesticide, which is lethally toxic to bees, was banned in the UK and EU in 2018.

It was approved for emergency use in the UK for a fourth year in a row in January, with the Government saying it could be used on sugar beet “because of a danger that cannot be contained by any other reasonable means”.

Farmers use the pesticide to kill aphids that can spread the beet yellows virus and the Government said it wanted to protect their livelihoods.

The move sparked anger from environmental groups who described it as a “death blow” to bees.

The OEP said it is looking into failures in relation to the process of making the decision, particularly in applying the “precautionary principle”, which helps decision-makers manage the risk of serious harm to the environment, when there is a lack of scientific certainty about the damage it could cause.

It is also looking at failures to meet nature conservation obligations.

Helen Venn, the OEP’s chief regulatory officer, said: “The investigation is looking at whether the appropriate assessments and considerations were undertaken in accordance with the law when those emergency authorisations were granted, rather than the outcome of the decisions themselves.”

The investigation is responding to a complaint by ClientEarth, an environmental legal charity.

It comes as conservation groups said analysis of official data shows neonicotinoids have been found in 85% of tested rivers in England, warning they could be damaging aquatic insects and the wider food chain.

Some of the highest rates were in areas where sugar beet farming is concentrated, The Rivers Trust and Wildlife and Countryside Link analysis found.

Responding to the OEP announcement, a Defra spokesperson said: “This Government has been clear that we will change existing policies to ban the use of neonicotinoid pesticides that threaten bees and other vital pollinators.

“The emergency authorisation decisions under investigation were taken under the previous administration. We will work constructively with OEP as they take forward this investigation.”