Wildlife groups claim the government would sound "a death knell for millions of bees" if it allows farmers to use banned pesticides to protect disease-ravaged crops.

Defra has approved an emergency temporary authorisation for a neonicotinoid seed treatment to be used on the 2022 sugar beet crop - most of which is grown in East Anglia.

The chemicals were banned by the EU in 2019 due to fears over their impact on the health of bees and pollinators.

But the temporary approval means - if strict conditions are met - it could be used to control aphids carrying virus yellows, a crop disease capable of devastating yields and damaging an industry which supplies more than 60pc of the UK's sugar.

The announcement was welcomed by farmers but provoked outrage from environmental campaigners, who said it betrays the government’s green promises and goes against the recommendations of its scientific advisers.

Joan Edwards, director of policy and public affairs at the Wildlife Trusts, said: “Less than two months ago the government adopted a legally binding commitment to halt the decline of wildlife by 2030 within its flagship Environment Act.

"The authorisation of this neonicotinoid flies in the face of this commitment and sounds a death knell for millions of bees and other insects.”

Eliot Lyne, chief executive of Norfolk Wildlife Trust, said allowing neonicotinoid use “will harm nature and could further pollute rivers", adding: "Every effort should be made to find alternative methods of pest control that don’t result in this level of damage to our wildlife, and in Norfolk where sugar beet is a major part of farming, it is vital.

“The urgent need to address biodiversity loss requires us to work at a landscape scale and in partnership with farmers by creating more, and better connected, wildlife-friendly habitat. Decisions such as these hamper this vital work.”

Fenland beet grower Michael Sly, who chairs the National Farmers' Union's sugar board, said he was "relieved that our emergency application has been granted".

“Virus yellows disease can cause huge devastation to a sugar beet crop, and only last year we saw crop losses of up to 80pc," he said.

"If pest pressure is that severe again, this authorisation will prove invaluable in fighting the disease and ensuring Britain’s sugar beet growers continue to have viable farm businesses.

"The sector continues to work hard to find viable, long-term solutions to this disease."

Defra said any temporary use will be "tightly controlled and only permitted in very specific circumstances when strict requirements are met".

Those conditions include that the chemical can only be used if the predicted level of virus is at or above 19pc of the national crop - and this threshold was not reached last time this step was taken at the start of 2021.

A Defra spokesperson said: "This decision has not been taken lightly and is based on robust scientific assessment."

They added that 12 EU countries including France, Denmark and Spain have granted emergency authorisations for neonicotinoid seed treatments since the ban.