From as far back at the 10th Century, the crop grew purple in India, the Middle East and Europe, with its origins traced to Afghanistan.

It was not until the 17th Century that they turned orange, when patriotic Dutch growers favoured the colour - as used on the national flag.

Now, the purple carrot is back and will be selling at �1.19 for 500 grams compared with �1.09 for its orange cousin in selected Sainsbury's stores from mid-July.

Rainbow veg

The carrots, called Purple Haze, only have the new livery on the outside - they remain the more familiar orange on the inside.

The store is also looking at introducing "a rainbow coloured bunch", with green, yellow, red, white and even black vying for room next to the Sunday roast.

The purple carrots are being grown at a farm in Yaxley, Suffolk.

Dr Wendy Doyle, of the British Dietetic Association, said that shoppers might be persuaded to try them because of their presentation value on a dinner-plate or in a salad.

Different pigments

And she said children might like the new and unusual carrot.

"Children love brightly coloured foods and it might just help to persuade them to try different vegetables," she added.

Carrots are extremely rich in betacarotene, a powerful anti-oxidant, which is found in the orange colour.

Purple vegetables contain pigments called anthocyanins, which also act as anti-oxidants.

Dr Doyle added: "Generally speaking, the brighter it is, the more anti-oxidant nutrients you are going to find, such as in mangoes, purple sprouting broccoli and peppers."