Chickens reared to supply major retailers are being kept in "horrifying conditions" on giant farms, an animal rights group has claimed.
Animal Equality UK said it uncovered "extreme suffering" at three Moy Park farms in Lincolnshire while carrying out a covert investigation.
The charity said carcasses were "left to rot for days".
Moy Park, which supplies products to supermarkets including Sainsbury's and Tesco, said it was investigating.
Investigators for the charity said they visited the farms on multiple occasions between February and April this year, entering through open doors and carrying out covert filming at night.
They said many of the birds were found with severe leg injuries, with some unable to stand.
"These birds were clearly suffering and should have been culled before they reached this advanced state of suffering," the group said.
Footage released by the charity also claims to show dead birds among the flocks at all three farms.
"Some of these carcasses had clearly been there for some time, bringing into question whether the required twice daily flock checks were indeed being carried out," it added.
Campaigners said they also found an accumulation of faeces and many of the birds were panting and had reddened skin at the rear.
Animal Equality UK executive director Dr Toni Vernelli said the investigation found birds being kept in "utterly dismal conditions", with up to 30,000 birds to a shed.
A spokesperson for Moy Park, which is one of the largest suppliers of poultry products in the UK, said: "We have a zero-tolerance attitude toward anything that jeopardises the health and welfare of our birds and we are fully investigating these allegations.
"We have robust processes in place to carefully monitor the welfare conditions for our birds and we have regular independent audits, taking corrective action with our farming partners if required."
In a statement, Sainsbury's said: "All our suppliers are expected to meet our high welfare standards and we are investigating this footage."
A Tesco spokesperson said: "We have strong procedures in place to ensure the welfare of the animals in our supply chain.
"We are working closely with our supplier to fully investigate this."
All three farms are certified by the Red Tractor farm-approval scheme, which requires produce to be "traceable, safe and farmed with care".
Ms Vernelli said: "Images of distressed birds in giant double-decker sheds will be a shock to many consumers who buy British, Red Tractor-certified meat thinking they can trust its animal welfare standards.
"Yet the truth is, the unnatural conditions chickens are forced to endure in these vast sheds are utterly dismal."
According to the charity, none of the farms had perches inside the sheds, violating Red Tractor's requirements.
A spokesperson for Red Tractor said: "Our own routine auditing of these farms had identified some breaches to our high standards, and we have been working with them to ensure they put the necessary processes in place for them to remain Red Tractor-certified."
Breaches found at one of the farms have already been resolved, while the other two are still within their 28 days to fully comply, they added.