Chickens reared by firms supplying major supermarkets spend their final days in “chronic pain”, an undercover investigation has found.

Dubbed “Franken-chickens” by animal welfare campaigners, these birds grow at an unnatural rate to satisfy Britain’s billion-a-year appetite for chicken.

Today, most UK factory farm chickens are from breeds developed to gain 95 grams a day.

They are slaughtered at 35 days old. In 1950 it was 16 weeks.

Animal welfare experts say the rapid weight gain often causes heart attacks and leg disorders.

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A huge flock of chickens was found crammed together

Video from Beauchamp Farm near Gloucester and Lower Cleeve Farm, Herefordshire, shows hens collapsing under their own weight.

Some were unable to feed properly, some had sores and were covered in faeces. Dead birds were left lying among the flocks.

The farms supply chickens to Tesco, Sainsbury’s and Lidl.

Tesco claims all assurance scheme and legislative requirements were being met.

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One of the 'Franken-chickens' seen in the charity's vid
The chickens cannot get up and escape their own filth

Conor Jackson, CEO of Open Cages, the charity that obtained the footage in July and August said: “The animals... are clearly in severe pain.”

Tesco now buys some of its chicken from indoor units that give slower-growing birds more space.

But this “room to roam” meat is only a small part of sales.

Fifty scientists have written to Dave Lewis, Tesco’s chief executive, urging him to sign a “better chicken commitment”.

The live birds live among the dead

Waitrose, M&S, Greggs, KFC, Nando’s and Pizza Express have already signed the commitment, which requires them to switch to slower-growing breeds and give birds more space by 2026.

In France, all the biggest retailers have now signed.

Andrew Knight, professor of animal welfare at the University of Winchester, said the footage showed “chickens with severe lameness... experiencing very substantial pain and stress”.

Tesco said it had begun an investigation after being made aware of the video and added: “We require all our suppliers to uphold high animal welfare standards.”

The charity said the animals were clearly in 'severe pain'

Sainsbury’s said: “The welfare of our animals is extremely important and we are investigating this footage from Lower Cleeve Farm.”

Lidl said: “Our fresh chicken is Red Tractor assured and adheres to stringent animal welfare and food hygiene standards.”

Avara Foods, which processes chicken from Beauchamp farm, said the footage showed the “overwhelming majority of the birds are clean, healthy and have easy access to food and water”.

Hays Farms, which owns Lower Cleeve Farm in Herefordshire, was unavailable for comment.