Selling potatoes and carrots still caked in mud doubles their shelf life and cuts their cost, according to Asda, which is offering bags of unwashed potatoes next to standard washed product.

It claims the mud acts as a natural preservative, preventing light from reaching the surface of the vegetable while also helping to keep it cool. Unwashed potatoes and carrots can last up to three weeks longer than their washed equivalents, while cutting production costs, waste and water use.

Asda's unwashed white potatoes cost 73p for a 2.5kg bag compared with £1.12 for the washed equivalent. A spokesman for the NFU said it was a great idea to supply muddy veg and hailed a change in consumer attitudes towards perfect-looking food. The retailer said the weight of each bag was adjusted so that customers paid just for the veg, not for the mud. It claimed sales of unwashed potatoes have jumped 20% since they were introduced last month.