Broilers can suffer various illnesses in their short lives
The animal charity says 800 million "broiler chickens" bred in the UK every year suffer painful health problems in cramped conditions.
Broilers regularly suffer illnesses such as sudden heart failure, leg pain, ammonia burns and skin infections, the RSPCA says.
Animal activists are angry that farming good practice labels demand less space for chickens than government guidelines. And they say even the government is too lenient.
RSPCA scientific officer Caroline Le Sueur told BBC News Online that chickens could be given more space without increasing the price of meat.
Ms Le Sueur said with the huge profit margin for supermarkets, it was retailers - not farmers - who should bear the costs of improving conditions.
'Systematic cruelty'
She added: "Consumers have traditionally been largely unaware of the suffering, but now they have a vital role to play in improving the lives of the birds," she said.
"Shoppers can influence animal welfare standards by the food they choose and the pressure they put on retailers to demand an end to systematic cruelty.
"Broiler chickens typically end up having considerably less space than an A4 piece of paper to move around in, even less than the space battery hens must have from 2003.
"Yet many consumers are unaware of the misery commonly endured by broilers."
The RSPCA report, "Behind Closed Doors", says that because broilers are bred for maximum meat yield in the shortest possible time, they now grow four times faster than egg-laying hens.
They are slaughtered after six weeks and are routinely given just one hour of darkness in which to rest per day to encourage growth.
High standards
About 100,000 die each day before slaughter because of the strains placed on their young bodies, the report says.
Peter Bradnock, chief executive of the British Poultry Council, told BBC News Online the RSPCA had wrongly put "the worst possible complexion on the industry".
"They are trying to bash UK standards which are high compared to the rest of the world."
"There are much greater differences in welfare standards in terms of some of the other countries which are exporting chicken to the UK in big volumes such as Brazil, Thailand, and Croatia."
Mr Bradnock said the government guidelines had not been updated since the 1980s since when vast improvements had been made in welfare issues such as temperature, feeding, and air conditioning.