Rival brand L'Oreal UK is said to have reported the campaign after they believed it was misrepresenting the product.
L'Oreal believed that the image of Portman had been photoshopped to exaggerate the effect of the mascara on eye lashes.
Christian Dior has since admitted to "post-production techniques" being used on the image but denied the accusations of misrepresentation.
"[We] did not consider that the post-production techniques went beyond the likely consumer expectations of what was achievable using the product," a spokesperson said.
The company also said that the image was intended to promote lipstick but a later decision was taken to use it for a mascara campaign.
However, the ASA did confirm that no consumers had complained about the ad exaggerating the effects of the mascara.
The ASA added that Portman's image was digitally retouched to "separate/increase the length and curve of a number of her lashes and to replace/fill a number of missing or damaged lashes, for a more stylised, uniform and tidy effect".
They have ruled that the ad cannot run in its current form as the ad's claims alongside the image of Portman may mislead consumers.