Wildlife campaigners are threatening legal action against the Government over its decision to temporarily lift the ban on harmful pesticides so East Anglia's farmers can use them on sugar beet.

Lawyers for the Wildlife Trusts have contacted environment secretary George Eustice to ask him to demonstrate the decision was lawful, or they will launch a legal challenge to the move.

The pesticides, known as neonicotinoids, are banned because of evidence they harm bees but farmers can apply for emergency use of the chemicals.

Earlier this month, the Government granted an application for an emergency authorisation for use of a product containing the neonicotinoid thiamethoxam for the treatment of sugar beet seed in 2021.

The Wildlife Trusts, a federation of independent conservation charities covering the UK, said that in 2018 the UK expert committee on pesticides refused a similar application because of unacceptable environmental risks.

Craig Bennett, chief executive of The Wildlife Trusts, said: "We are preparing to take legal action unless the Government can prove it acted lawfully.

"The Government refused a request for emergency authorisation in 2018 and we want to know what's changed. Where's the new evidence that it's OK to use this extremely harmful pesticide?

"Using neonicotinoids not only threatens bees but is also extremely harmful to aquatic wildlife because the majority of the pesticide leaches into soil and then into waterways.

"Worse still, farmers are being recommended to use weed-killer to kill wildflowers in and around sugar beet crops in a misguided attempt to prevent harm to bees in the surrounding area. This is a double blow for nature."

Pamela Abbott, chief executive of Norfolk Wildlife Trust, said: "The evidence of the threat to insects from the use of pesticides was behind the decision of UK Expert Committee on Pesticides in 2018 to refuse an application for the use of neonicotinoid pesticides.

"Norfolk Wildlife Trust does not believe that there has been any change in the evidence since that time and supports the questioning of the Government’s decision to allow the emergency use of a banned neonicotinoid for sugar beet. In our view there remains a need for urgent action to restore the abundance of our insect populations which have suffered drastic declines, with 41pc of all insect species now threatened with extinction."

More than 56,000 people have signed a petition by the Wildlife Trusts to ask the Prime Minister to overturn the decision, he added.

A Department for Environment, Food and Rural Affairs spokesman said: "Emergency authorisations for pesticides are only granted in exceptional circumstances where diseases or pests cannot be controlled by any other reasonable means. Emergency authorisations are used by countries across Europe.

"Pesticides can only be used where we judge there to be no harm to human health and animal health and no unacceptable risks to the environment. The temporary use of this product is strictly limited to a non-flowering crop and will be tightly controlled to minimise any potential risk to pollinators."