One in three standard supermarket chickens have suffered skin burns caused by standing in their own excrement, investigators claim.
The RSPCA warned there are worrying levels of the condition – known as hock burn and caused by the ammonia in waste.
Animal welfare investigators from AF News Agency claim 37% of 215 Marks & Spencer Oakham chickens and 35% of 377 Sainsbury’s own-brand chickens they inspected had hock burns.
But many supermarkets sell chickens with the lower leg removed, meaning they cannot be inspected.
Critics say this means the problem could be even more rife.
RSPCA Head of Farm Animals Dr Marc Cooper said: “Hock burn is a widespread problem in meat chickens.
"It is primarily caused by prolonged contact with wet litter, the material covering the floor.
"There are many factors that contribute to poor litter, such as overcrowding.
"Ideally, there should be no hock burn.
“Previous research has shown a worrying level of hock burn among standard chickens and indicates the level is much higher when birds are poorly managed.”
He called for compulsory checks on all chickens and said birds raised in line with RSPCA standards are more than five times less likely to suffer from hock burn.
A spokesman for M&S said: “Hock-marking is a challenge for all retailers and we work closely with our farms to reduce it.”
Sainsbury’s said: “Suppliers are expected to comply with our code of practice and meet the high welfare standards our customers would expect.”
Meanwhile, PETA activists have obtained footage showing thousands of broiler chickens being raised for meat in huge windowless sheds on a farm in Norfolk.
Images show the birds packed so tightly they can barely move.
It is not known what outlets the farm supplies.
PETA Foundation director Elisa Allen said: “The suffering exposed is not the exception, it is almost certainly the rule.”
The British Poultry Council declined to comment on the footage.
We passed details of the farm to the RSPCA, which said: “Shoppers who want higher welfare chicken can look for the RSPCA Assured logo, free-range or organic.”