Dozens of birds collapsed under the weight of their unnaturally large bodies and were left unable to stand as a result of such ill treatment.
Chickens were filmed being deliberately kicked and stepped on in the harrowing footage, with workers caught callously breaking their necks and leaving them to convulse amid the flock.
Workers were also shown to be throwing dying birds onto a pile where they were left to suffer for hours as they cleared the shed for slaughter.
Dead birds were left to rot among the living, leading to cannibalism on at least one of three farms in Northamptonshire which rear chickens for Faccenda, one of the UK's largest chicken companies, and a supplier for major outlets including Nando's, Asda and Lidl.
The farms, at Evenley, Pimlico and Helmdon, are all certified by Red Tractor, which claims that "animal health and welfare is at the heart" of its standards.
However, in the footage released by animal protection organisation Animal Equality, some of the birds could be seen to be struggling aged just seven days old.
Investigators made multiple visits to the farms between January and March 2019, after receiving a tip-off about poor conditions. On every visit, they found chickens unable to stand dead birds left to rot inside the overcrowded sheds.
After discovering bin bags full of dead birds inside one shed Evenley Farm, they installed a hidden camera to record how long the carcasses were left among the living.
Footage captured workers subjecting the birds to horrendous cruelty, including breaking their necks and stepping on them.
Animal Equality's UK director, Dr Toni Vernelli, said, "Sensationalist headlines about American chlorinated chicken would have us believe that British birds live a life of luxury, but these harrowing scenes reveal the truth.
"Crammed inside crowded sheds and bred to grow so unnaturally large that their joints and hearts can't cope with the strain, chickens on British farms suffer every minute of their lives.”
All of the footage has been passed to the RSPCA, Red Tractor and Defra's Animal and Plant Health Agency.
Companies looking into allegations
A spokesman for Nando's said, "Animal welfare is as important to us as it is to customers, which is why all of our chickens are barn-reared in the UK to Read Tractor standards.
"We expect all our suppliers to operate to high standards and we are disappointed with the footage.
We will be working with the supplier to get to the bottom of these allegations."