The 37 farms that supply fresh milk to M&S have achieved RSPCA Assured certification and all M&S fresh milk packaging* now carries the RSPCA Assured logo.
Together the farms produce 150 million pints of milk a year.
Clive Brazier, RSPCA Assured chief executive, said: “Thanks to M&S thousands more dairy cows will now have a better life, reared to RSPCA welfare standards. Not only that but - for the first time ever - consumers will be able to buy RSPCA Assured labelled milk in a high street retailer.
“This is a major step forward for improving dairy cow welfare and we hope other retailers will follow suit.”