Featuring the strapline, For the Love of Food, the ads explain that all eggs used as ingredients in M&S products are now free range Â a first in the industry.
New PoS material instore will also highlight M&S's close relationship with farmers and profile selected suppliers.
M&S business unit director for food, Justin King, said: "This is a real milestone in food retailing. We're committed to producing the highest quality food in the most compassionate way and this means taking a serious stance on animal welfare issues."
The chain uses 250 million eggs a year as ingredients, so the free range initiative has taken several months to implement, said a spokeswoman.