Purple carrots - dark on the outside, orange on the inside - are set to appear in shops this summer, while green, yellow, red, white and black varieties may follow.

Growers say the change of colour is entirely natural, as carrots occurred in variety of different colours before the Dutch bred an orange variety in the 16th century that became an overwhelming commercial success. Temple drawings show that as long ago as 2000BC the Egyptians had a purple-coloured plant thought to be the carrot.

Researchers at the University of Wisconsin report that by the 1300s, purple and yellow varieties of carrot had spread from origins in Afghanistan to Europe and China. Red and white carrots were also grown.

Orange carrots contain high levels of betacarotene, a pigment which is a form of vitamin A, that encourages good vision in dim light and healthy skin. Purple carrots also contain anthocyanin, which may help prevent heart disease by slowing blood clotting.

The first crop of dark purple carrots is being grown at Yaxley, Suffolk, and will be available in supermarkets from the end of July. Sainsbury's, which is stocking the carrots called Purple Haze, said they were looking at introducing a "rainbow coloured bunch" soon, but a spokeswoman said no other colours had been planted yet.

Modern agriculture has led to a loss of genetic diversity. According to a Worldwatch Institute report in 1999, less than 20% of the vegetable seeds available at the turn of the last century were commercially grown 100 years later.

Over 90% of carrot varieties have been lost. In 1903 there were 287 varieties of carrots being grown, but 80 years later there were only 21 types of carrot seed in the US national seed storage laboratory, according to the World Resources Institute.

At Sainsbury's, they are hoping for a more prosaic result. Russell Crowe, a buyer for the supermarket chain, said: "Hopefully this unusual colour will influence children to eat more vegetables, while parents can rest assured there is nothing artificial about the carrots."