Dead birds were left to rot amongst the living while others were covered in their own filth at three of the UK's largest poultry providers, who supply top chains including Asda, Tesco, Lidl, and KFC.

That is according to a secret investigation conducted by Viva!, Europe's leading vegan campaigning charity.

Thousands of chickens were forced to live in "abhorrent conditions", with those that died "trodden into the ground and covered with litter", according to reports.

The charity has since warned that all three sites, which are all Red Tractor approved, are a significant threat to public health. 

Tesco's is said to have launched an immediate investigation after hearing the findings, which one Veterinary Professor described as "highly disturbing".

All three farms produce broiler chickens, otherwise known as 'meat chickens', birds that are selectively bred to rapidly gain weight. 

Such methods are commonly used in the poultry industry, but at Haywood Poultry farm in Herefordshire, investigators reported that the chicken's underdeveloped bones collapsed under their own body weight, leaving many "crippled or lame". 

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Haywood Poultry is under contract with Avara Food's - Tesco's biggest supplier of poultry meat. 

In Taunton, Somerset, a farm that supplies Hook 2 Sisters, who produce one third of all poultry products eaten every day in the UK, similar conditions were reported. 

At Churchinford Poultry, a spokeswoman for Viva! said that chickens were found collapsed on the "ammonia-rich litter that covers the shed floor with painful hock burns marking their raw skin".

At a Derbyshire farm under contract to Moy Park, which sells chicken to Tesco and Sainsbury's, footage revealed a high number of bald birds, most likely as a result of aggressive feather-pecking, a sign of distress amongst chickens.

Overbrook Farm's 14 sheds were said to be filled with chickens "as far as the eye could see".

Viva!’s Head of Investigations, Lex Rigby, said: "Clever marketing tactics continue to mislead consumers into believing the UK has some of the highest animal welfare standards in the world, but the reality is a far cry from what we see in the adverts.

"The farms in our investigation supply major supermarkets, restaurants and food outlets and are considered industry standard. Sadly, none of the conditions our investigators filmed are illegal – which makes the findings even more disturbing."

Both supermarkets and their supplier have since hit back at the allegations made by Viva!.

A Tesco spokesperson said: "We require all our suppliers to uphold high animal-welfare standards. Any claims of suppliers falling below these are unacceptable. We immediately began an investigation of these farms as soon as we were made aware of the allegations.

"Independent audits of the three farms found that assurance scheme and legislative requirements were being met at the time of inspection, but we will continue to monitor the situation closely and further audits will be carried out."

A Lidl spokesperson said that they were committed to ensuring continued progress amongst its suppliers, adding: "All our chicken complies with nationally recognised standards, including Red Tractor Assured, RSPCA certification and Organic Farmers and Growers. There are strict controls in place on how birds are handled and cared for, with compliance ensured through regular audits."

A Moy Park representative said that "the farm was independently audited immediately and was certified as fully compliant with all animal-welfare standards".

A Red Tractor spokesperson said: "Protecting animal health and welfare is a top priority. As soon as we were made aware of the footage, an investigation was launched to substantiate whether they presented an accurate representation of the farms’ management and implementation of our standards. All sites were found to be well managed and compliant with our scheme standards."

Avara released a statement that said: "The welfare of the animals in our care is of the highest importance to both us and our customers. After being made aware of this footage, we undertook a spot audit of the farm in question, as did Red Tractor. We were absolutely satisfied with the standard of bird welfare evident on the farm and Red Tractor found no major issues of concern."

A Hook 2 Sisters spokesman said: "We abhor any reports of animal suffering and condemn any breaches of the high standards we expect everybody to uphold.

"In this case, the allegations are untrue. This farm has been inspected by eight independent auditing bodies, and a recent veterinary surgeon’s report confirms no concerns have been highlighted at any point, including the last Red Tractor audit on July 22."