A Swindon supermarket has been fined £5,000 after making illegal claims about the health benefits of mangoes.
Asda staff held a protest outside the court
Asda pleaded guilty to contravening regulation 40 (1) and 44 of the Food Labelling Regulations 1996.
The company admitted it labelled the fruit as having "anti-oxidant properties to help to fight cancer".
Asda said it was a "genuine mistake". Swindon Borough Council took the chain to court after Trading Standards officers spotted the sign in June 2003.
Swindon Magistrates Court heard the sign read: "Mangoes are a great source of Vitamin C and beta-carotene, which are good for healthy eyes and skin.
"Try adding mango to smoothies, fruit salad or breakfast cereal."
The claim that the fruit may help fight cancer contravenes the regulations which state that any labelling claiming that a food prevents, treats or cures a disease is prohibited.
The store said the error was a "genuine mistake"
A charge under Section 4 (1) of the Cancer Act, 1939 was dropped after the council offered no evidence.
Outside the court, six Asda employees dressed as pieces of fruit demonstrated with banners which read "Healthy Not Guilty".
Spokeswoman Rachel Fellows said: "It seems crazy to us that... the undisputed benefits appear on Department of Health and Food Standards Agency websites but that we cannot echo those statements in our stores.
"We don't think it's right that you can state exactly why you should eat lots of fruit and vegetables anywhere except the place where people want to buy them. That's why we think the law must change, and now."
In a statement, the head of trading standards at Swindon Borough Council, Robert Taylour, said: "Current Government recommendations are that everyone should eat at least five portions of fruit and vegetables each day to reduce the risks of cancer and coronary heart disease and many other chronic diseases.
"We fully support this message, however, in this instance the claim went beyond the message of prevention and suggested a treatment or cure.
"It is clearly unacceptable for the public to be misled in this way."
Asda later said it was launching a campaign to change food labelling regulations.
A company statement said the firm was writing to Health Secretary John Reid - who has commented on the links between diets rich in fruit and vegetables and the reduced risk of cancer - to ask for his help in clarifying the law.