Marks and Spencer has committed to selling only 100% slow-reared fresh chicken, becoming the first retailer to do so.
The roll out of the 'Oakham Gold' line is part of the retailer's aim to become the market-leader in animal welfare standards.
And in line with the Better Chicken Commitment, the supermarket chain has moved to a Hubbard chicken breed, which will also benefit from higher welfare standards.
These include a multigrain diet, which has been designed to support slower natural growth and muscle development.
As the birds are slower reared they will be more resilient and robust, Marks and Spencer said, particularly to higher temperatures.
Birds will also have 20% more space alongside an enhanced environment, with more pecking aids and perches to encourage natural behaviours.
Andrew Clappen, M&S food technical director said: “When it comes to chicken, we want to keep raising the bar to improve welfare - it’s something we know our customers care deeply about and we do too.
“Improving animal welfare is an important part of our trusted value commitment – we are introducing slower-reared, higher-welfare chicken offering better quality and better flavour for our customers.”
Mike Baker, CEO of RSPCA Assured, said the move was the "biggest positive change to chicken farming in a generation".
"It’s a landmark achievement for animal welfare, which we hope will set a leading example for other retailers," he added.
“The positive impact this will have on M&S’ chickens is enormous, allowing these alert and curious birds more comfort and freedom to enjoy their natural behaviours."