The two-page ad for Revitalift Repair 10, which appeared in September, featured a black and white close-up photograph of Weisz and a list of the ten properties of the cream, including reducing the appearance of wrinkles.
The Advertising Standards Authority (ASA) said it had upheld a complaint by Jo Swinson, a lawmaker for the junior government coalition partners, the Liberal Democrats, and a campaigner on improving body image.
"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," an ASA ruling said.
"We therefore concluded that the image in the ad misleadingly exaggerated the performance of the product in relation to the claims 'Skin Looks Smoother' and 'Complexion Looks More Even'."
L'Oréal said cosmetics firms were entitled to present their product in the best possible light, adding that Weisz had been professionally styled and made up and photographed under ample light and with a soft focus.
Swinson welcomed the ruling, under which L'Oréal must not publish the ad in its current form again, saying: "The beauty and advertising industries need to stop ripping off consumers with dishonest images."