Undercover footage filmed over the past two months on a West Country chicken farm has been released by leading international animal protection organisation Animal Equality.
The footage shows more than 150,000 birds housed in four giant sheds in crowded, barren conditions.
Other distressing scenes include a worker kicking birds during daily checks inside the sheds; collecting ill birds and breaking their necks, with some still alive and flapping their wings as he carried them around.
Another showed workers violently catching and crating birds for transport to the slaughterhouse, including carrying birds by one leg in violation of Defra’s welfare code.
Dr. Toni Shephard, UK Executive Director of Animal Equality, said: "The birds were just a few days old when we first filmed, yet already hundreds of chicks were dying every day and the bins outside the giant sheds were full of tiny bodies, still with their yellow baby feathers.
"Just a couple of weeks later, the skips were fuller still and many of the birds were suffering from painful lameness. By our last visit, the sheds were so crowded it was difficult to walk through them.
"Some birds were on their backs slowly dying as they were unable to stand up, while others had large sores from constantly sitting in the ammonia-soaked litter as they were in too much pain to walk."
Shephard added: "These distressing scenes suggest we are not ‘leaders in animal welfare’ as claimed in the current row over US imports."
According to the organization, modern breeds of chicken have been selected to gain weight fast, which maximizes profits at the expense of welfare. Their young legs struggle to support an adult-sized body and their small hearts are put under enormous pressure, leading to an on-farm mortality rate of three - five per cent.
"That equates to tens of millions of birds not surviving to the slaughter age of six weeks. Yet the lives of these animals have such little value that it is more profitable to suffer these losses than improve conditions."
Animal Equality has passed this evidence to the Animal and Plant Health Agency (APHA) for investigation.