Fastfood chain McDonald's has been told not to repeat an approach to advertising its fries, after a magazine advert was judged by the UK's advertising watchdog to be misleading.

The advert in question was headlined "The Story of Our Fries. (End of Story)" and showed a potato in a McDonald's fries box. The advert also said: "First, we take the potatoes. (The Russet Burbank, Shepody and Pentland Dell are the only varieties we use because they're the perfect shape and especially good for frying.) We peel them, slice them, fry them and that's it. This simple process might not make for a very long story, but it certainly makes for irresistibly long fries."

The UK's Food Commission objected to the advert on the grounds that significant amounts of salt were added, which had health implications. Members of the public also objected that the fries were flash fried in beef tallow, frozen while partially cooked, flown around the world and then re-deep fried; that sugar solution, including a dextrose solution were added at certain times of the year; and that fries could contain gluten from other products because of cross-over in cooking oil.

McDonald's argued that the advertisement was intended to explain to consumers that its fries were made from real, not reconstituted, potato and that the copy was not intended to be a literal and comprehensive statement of all the processes involved.

Regarding the complaint from the Food Commission, McDonald's said that salt was added immediately before serving only and that that was irrelevant to how the fries were produced. The company also said that, when requested, fries could be provided without salt.

McDonald's said that dextrose and other sugars were naturally present in potatoes, but that at low levels these produced paper-white fries that were unattractive to the customer. A low-concentrate dextrose solution was therefore added to bring colour consistency and McDonald's said that had no significant nutritional impact or perceivable effect on taste.

The Advertising Standards Authority said it was satisfied that McDonald's fries did not contain gluten from cross-cooking and that the company did not use beef tallow.

It considered, however, that because the advertisement contained claims such as "End of story" and "...and that's it" most people would understand that it described the complete process of how the company produced its fries.

The ASA said that because material parts of the process were omitted from the advertisement, it had concluded that the advertisement was misleading and told the advertisers not to repeat the approach.