Horrific pain and suffering inflicted on chickens bred at farms supplying major UK supermarkets including Tesco, Sainsbury’s and Aldi has been claimed after activists released undercover filming.

Videos taken just before slaughter shows birds are struggling to walk as their legs buckle and they collapse under their weight while they flap their wings helplessly.

Bred to grow faster, they often become lame, suffer heart problems and skin diseases.

Other clips show the animals cramped in indoor farms with barely any room to move, with some pressed against walls and other seemingly standing on each other, while undercover campaigners said they also found bins full of dead bodies or other dead animals left in indoor farms for hours, sometimes overnight.

The chickens were filmed at farms operated by two of the UK’s largest suppliers, Hook 2 Sisters in Devon and Moy Park in Lincoln, whose customers include chains such as Tesco, Aldi and Sainsbury’s.

Scenes “were consistent with significant pain”, said Andrew Knight, professor of animal welfare and ethics, and founding director of the centre for animal welfare at the University of Winchester.

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He added that the Moy Park footage showed “meat chickens with serious mobility problems”.

“Despite vague PR statements of ‘taking welfare seriously’, footage of animals collapsing under their weight is uncovered practically every week,” said Open Cages CEO Connor Jackson, the campaign group that released the undercover videos.

The animals belong to very specific breeds called Ross 308 and Ross 708, the most common fast-growing breeds of chicken, which according to Open Cages account for about 70 per cent of the entire production of chicken meat in the European Union.

Fast-growing chickens have been selected artificially through the years so that broilers would grow unnaturally quickly, allowing farmers to maximise profits.

The birds can take about 35 days to achieve the target weight for the slaughter of 2-2.5kg, while a slow-growing specimen can take up to 70 to 90 days, a 2016 European Commission report found.

The report said that chicken growth rates have quadrupled in just 60 years, with birds taking just 30 days to achieve the weight of 1.5kg today – compared to 120 days in the 1950s.

“If people grew this fast, a five-year-old child would weigh 150kg,” said Mr Jackson.

The chickens’ extraordinarily fast growth rate can cause their premature bodies to struggle to handle the weight.

The report said that leg abnormalities, skin diseases and poor bone structure are common among fast-growing chickens, while the most common cause of death was sudden death syndrome (SDS), which often occurred when the chicken couldn’t take in as much oxygen as its oversized body required.

“Sadly, this footage is a reflection of modern broiler production where demand for fast growth – achieving the greatest meat yield in the shortest time – continues to be the primary focus,” the RSCPA said in a statement.

Mr Knight, the veterinarian, said the chickens at the farms were also “grossly crowded” and unable to exercise natural behaviours.

Mr Jackson also criticised that supermarkets’ packaging as “misleading”, displaying pictures of idyllic farms and chickens walking freely on packages of factory chicken.

Research from the RSPCA found the most popular supermarkets encourage consumers to buy chickens that have suffered in industrial-style systems through labels such as “higher-welfare” or images suggesting birds were free-range.

“These scenes are all too common within this industry,” says Claire Palmer, founder of Animal Justice Project, commenting on the footage.

An option for many suppliers and supermarket and restaurant chains has been to sign up to the Better Chicken Commitment (BCC), an initiative launched by the RSPCA.

BCC signatories – which include M&S, Waitrose, Nestle and KFC – commit to adopting new minimum standards, including higher welfare breeds and lower stocking densities, by 2026.

However, Ms Palmer believed the commitment to be a very small step.

“Our cameras have exposed the gross suffering present on so-called high welfare farms – harsh and cramped conditions, abuse and deformities,” said Ms Palmer. “Many ‘slow-growing’ chickens never even see the outside. There is no humane way to farm chickens on this current scale.”

She says the only way to stop it is for consumers to choose vegan products and for governments to stop subsidising the industry and support farmers in their move towards a plant-based food system.

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A spokesperson for Hook 2 Sisters said: “Allegations in relation to a two-minute piece of footage taken at Swanham’s farm are untrue, paint a distorted picture of the reality on the ground and ignore the clinical facts that we have provided.

“This particular crop of birds was clinically assessed three times over 30 days by veterinary officials and no welfare issues were identified at any point. Abattoir inspection reports logged independently by the FSA for this flock show 1.22% rejections, of which 0.02% were for lameness.

“This farm was inspected for compliance by vets on Friday March 1st 2019 and again on Tuesday March 4th 2019 by Red Tractor auditors. Again, both identified no welfare issues.

“Animal welfare is a top priority at our farms and something we take extremely seriously, as evidenced by the inspection regime outlined above. All our farms operate to UK and EU legislation and are independently audited and accredited to Red Tractor standards as a minimum.

“In fact our own welfare measures exceed this standard and our own policies require the birds to have access to natural light and environmental enrichment.”

A spokesperson for Moy Park said: “In regards to the leg issues / lameness you reference, lameness can affect all livestock and is caused by a number of factors (eg bacterial, viral, metabolic). It is not specific to any particular breed or rearing system.

“Moy Park has in place systems to carefully prevent, manage and reduce lameness, including health, environment and nutrition planning, supporting a team of highly trained and dedicated farmers. Where lameness occurs, these birds are removed from the flock.”

A Tesco spokesperson responded to the footage saying: “Any claims that our standards have not been met are always fully investigated and we are currently working with Moy Park to review this situation.”

An spokesperson for Aldi said: “We have strict welfare standards in place that we expect all of our suppliers to uphold and we are currently investigating these allegations.

“We are the largest retailer of RSPCA Assured chicken by volume and all the fresh chicken we sell meets either Red Tractor or RSPCA Assured standards.”

A Sainsbury’s spokesperson said: “We are committed to high standards of animal welfare. It is what our customers expect from us, and it is why we are the UK’s biggest retailer of RSPCA Assured products and the world’s biggest retailer of MSC certified products. While we share Open Cage’s commitment to improving animal welfare practices, we believe a different approach is more effective.