A little egg-stra for breakfast: Six double-yolkers for £2.75

M&S said the six packs of British free range eggs will go on sale nationwide this weekend, with each egg containing two or more yolks.

Double yolkers are thought to occur in less than 0.1% of eggs - or one in 1,000 - according to the British Egg Information Service.

They tend to come from young hens whose hormone system has not yet fully developed.

It is not unusual to find more than one double yolker in a box because all the hens in a flock are the same age.

But to ensure a full box of six, M&S suppliers use a method called “candling” to individually inspect each egg by hand in front of a bright light to show the shadow of the yolks inside.

The yolk is the “nutritional centre” of the egg and contains high quality essential fats, vitamins and minerals.

The vitamins include B12, B6, A, K, E and folate, and yolk is one of the few foods to naturally contain vitamin D.

Yolks also contain most of the egg’s calcium, magnesium, potassium, sodium, selenium and iron.

M&S product developer Ali Rodham said: “We’re really excited to be selling double yolkers. We think they’re cracking, and we’re sure our customers will too.

“Perfect for people who think the yolk is the best bit of the egg, double yolkers are not only delicious, but they are healthy too.”

British Free Range Double Yolkers are available for £2.75 for a box of six.