M&S claimed the Croloaf is the UK's first croissant/loaf hybrid and that, at 114 calories a slice, it is healthier than a normal 340-calorie croissant.
On sale now for £2.40, the product was described as “the perfect fusion of the French and British breakfast” by M&S.
The enriched pastry loaf, made with all-French butter, can be sliced like normal bread. But unlike a croissant, which needs to be eaten on the same day it's bought, M&S said the Croloaf tastes just as good when it's a few days old.
The supermarket was inspired to launch the Croloaf after seeing similar loaves in artisan bakeries in America, where Chicago business Beurrage is well known for its version.
The Croloaf comes weeks after Dominique Ansel, the French-born pastry chef who created the Cronut, opened his first bakery in London.
“Our croissant loaf is perfect for those who like the convenience of grabbing a slice of toast for breakfast but love the delicious buttery taste of croissants,” said M&S bakery spokeswoman Sadia Usman.
“Now they don't have to compromise and can enjoy the best of both.”
Last month M&S announced its take-out sandwiches will all contain extra vitamin D.