The two-page magazine ad, run in September last year, featured the Mummy star alongside various claims about the effects of using Revitalift Repair 10 from L'Oreal.
Jo Swinson MP challenged whether the ad was misleading as she believed the image of Weisz, who married James Bond star Daniel Craig last year, "had been digitally manipulated and therefore misrepresented the results that the product could achieve".
L'Oreal said that the ad was intended to "represent Rachel Weisz as favorably as possible and therefore every effort had gone into ensuring the most flattering set-up".
The cosmetics giant said that the actress had been professionally styled and made up before being shot by a professional photographer in a studio. The team also used 'soft focus' lighting in a black and white image to maximise the 'glamour' of the advert.
The ASA noted that L'Oreal's claims about Revitalift Repair 10 making skin appear plumper and more hydrated were "acceptable" for the product.
It also accepted that advertisers "were keen to present their products in their most positive light using techniques such as post-production enhancement and the re-touching of image".
But after L'Oreal supplied the ASA with information showing the level of post-production used on the image, the regulator ruled that it "had been altered in a way that substantially changed [Weisz's] complexion".
"Although we considered that the image in the ad did not misrepresent the luminosity or wrinkling of Rachel Weisz's face, we considered that the image had been altered in a way that substantially changed her complexion to make it appear smoother and more even," said the regulator.
"We concluded that the image in the ad therefore misleadingly exaggerated the performance of the product in relation to the claims."
The ASA said that the ad must not appear again in its "current form" and ordered L'Oréal to ensure that it did not use post-production techniques in future adverts in a way that "misrepresented what was achievable using the advertised product".
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