In glossy magazine shoots, retouching is de rigueur. “I have never yet seen, and you probably never will see, a fashion or beauty picture that hasn’t been retouched,” Mr. Hudson said.

In Britain, tabloid outrage over the use of unhealthily thin models in fashion shows and magazine spreads has been fueled by photos that have been altered to streamline models or celebrities. One often-cited example is a 2003 cover of GQ magazine in Britain on which the actress Kate Winslet appeared several sizes smaller than her actual self.

But some magazine editors say they are now overcompensating in the other direction. “I spent the first 10 years of my career making girls look thinner,” Robin Derrick, creative director of British Vogue, told The Times of London recently. “I’ve spent the last 10 making them look larger.”

On retouching, even Ms. Swinson acknowledged that “a little bit is necessary to make a good photo.” Under her proposal, all advertising photos would be rated, perhaps on a scale from 1 to 4, depending on the degree of retouching. A “1” might involve only altered lighting, for example, while a “4” might warn of cosmetic changes via Photoshop, she said. And the label would have to include an explanation of the changes. “If people knew they had to describe what they had altered, it might make them less likely to do it,” Ms. Swinson said.

Unlike Ms. Boyer, Ms. Swinson said she thought such a system could be put in place without legislation. She said she hoped to work through the Advertising Standards Authority, which monitors the content of ads in Britain, to encourage advertisers to adopt it.

The Liberal Democrats have been trying to put pressure on the standards authority, which is financed by advertisers, with a Web site urging consumers to complain about examples of strikingly altered ads. The site has highlighted a campaign this summer for Olay cosmetics, in which the model Twiggy appears to have been given a younger woman’s skin, and an ad for Chanel Coco Mademoiselle perfume in which the actress Keira Knightley sports an apparently enlarged bosom. Ms. Knightley has joked about the featuring of such enhancements in ads for the movie “King Arthur,” in which she starred.

Matt Wilson, a spokesman for the standards authority, said that out of 26,000 complaints from the British public last year, five involved airbrushing. He said the agency already had sufficient tools to deal with misleading ads of all kinds, not just those that involve retouching.