Swinson, Lib Dem MP for East Dunbartonshire, complained to the advertising watchdog that a magazine ad for Lancôme’s Teint Miracle foundation and another magazine ad for Maybelline’s The Eraser foundation were misleading, because of the alterations. Both ads were created by US-based agency Gotham.
After investigating, the ASA upheld her complaints that the ads were misleading and banned them in their current form.
L'Oréal admitted to the ASA that "post-production techniques" had been used in the Maybelline ad to place "blocks" over parts of Turlington’s face to "differentiate between areas the product had been applied to and areas it had not".
The company also said it had used post-production to "digitally retouch" parts of the model's appearance. However, it said that despite the use of digital technology, the ad "accurately illustrated the results the products could achieve".
The cosmetics firm said the images in the Lancôme/Julia Roberts ad had been taken by Mario Testino, acclaimed for his flattering photos, and used lots of lighting to reduce the appearance of imperfections.
While providing details of the post-production techniques used, L'Oréal said they did not relate to characteristics directly relevant to the performance of the product.
The ASA said it noted the factors outlined by L'Oréal, but while it had provided details on the techniques used, it did not provide information that allowed the watchdog to see the effect those enhancements had on the final image used in the Roberts ad.
The ASA said: "On the basis of the evidence we received, we could not conclude that the ad image accurately illustrated what effect the product could achieve and that the image had not been exaggerated by digital post-production techniques,"
A spokeswoman for Lancôme said the brand did not believe its ad exaggerated the effect of using the product.
She said: "We are disappointed to learn that the ASA has adjudicated against our press advertisement for Teint Miracle. The product has been proved scientifically to provide luminosity to the skin.
"We do not believe that the ad exaggerates the effect that can be achieved using this product.
"In consumer tests among over 100 women, 77% agreed that Teint Miracle made their complexions look radiant and luminous."
In a statement, Maybelline said: "We are disappointed to learn that the ASA has adjudicated against our press advertisement for The Eraser.
"The advertisement features Christy Turlington who has beautiful skin and features a product that has been proved scientifically to conceal imperfections.
"Even though the ad features an obviously illustrated effect, some lines are still clearly visible beneath the illustration and we do not believe that the ad exaggerates the effect that can be achieved using this product. In consumer tests among 253 women, 78% agreed that The Eraser provided flawless coverage."
The ASA said it accepted the images in the Turlington ad showed wrinkles and blemishes where the foundation had been applied. However, the watchdog said the area round the model's left eye was "digitally retouched" and the supporting text drew particular attention to this area.
The ASA ruled: "The combined effect of the image and the surrounding text was to suggest that the product could have a significant impact on the appearance of imperfections in the skin."
The regulator said the text claims were broadly consistent with the results of consumer testing, "However … information provided regarding the digital retouching of the image was insufficient to establish if the difference between the 'blocks' was an accurate representation of the results the product could achieve."
Swinson said the ruling demonstrated that the ASA acknowledged the dishonest and misleading nature of excessive retouching.
She said: "Pictures of flawless skin and super-slim bodies are all around, but they don't reflect reality.
"With one in four people feeling depressed about their body, it's time to consider how these idealised images are distorting our idea of beauty.
"Shockingly, even the ASA weren't contractually allowed to see the pre-production photo of Julia Roberts.
"It shows just how ridiculous things have become when there is such fear over an un-airbrushed photo, that even the advertising regulator isn't permitted to see it.
"Excessive airbrushing and digital manipulation techniques have become the norm, but both Christy Turlington and Julia Roberts are naturally beautiful women who don't need retouching to look great. This ban sends a powerful message to advertisers – let's get back to reality."
This followed a similar dismissal of 13 complaints about Cole's L'Oréal Elvive shampoo ad in 2009, after consumers accused the 'X Factor' judge of misleading the public because she had hair extensions.
However, in 2007, the watchdog did ban a make-up ad by L'Oréal claiming "up to 60% longer lashes", after discovering that the star, Penelope Cruz, was wearing false lashes.