The TV advert, for the perfume Coco Mademoiselle, depicted British star Knightley being photographed on a bed, with the photographer unzipping her clothes and revealing her shoulders and part of her back.
Dressed only in a bedsheet, she crawls towards the photographer, before lying on her back. The man then appears about to kiss her when she put a finger to his lips, and says "lock the door".
A viewer complained that the "overtly sexual" ad should not have been shown during programming that was likely to appeal to children, after they saw it during a screening of the animated film Ice Age 2.
Chanel, maker of Coco Mademoiselle, said that its founder Coco Chanel was "known for being a strong, independent woman", and the ad was meant to "reflect her spirit and embody the fragrance itself".
The firm said that it was chosen to air during Ice Ace 2 because the film has "sharp humour and involved recognised celebrities as voice actors; it therefore also had a clear appeal to adults".
Chanel also said that there was no nudity in the ad apart from Knightley's bare shoulders, and the sexual theme was "playful and sensual", in keeping with most perfume ads.
However, the Advertising Standards Authority considered that the context of the photo shoot in the ad and the scenes depicted "involved sexually suggestive content".
"We noted that the photographer was directly involved in unzipping the actress's garments and that there was a suggestion that she was naked aside from a bedsheet. We also noted that there was clear sexual tension between the pair and that they appeared about to kiss on the bed," said the regulator.
"We noted that Ice Age 2 was of particular appeal to children. We considered the ad was suitable for older children, but that the sexually suggestive material was unsuitable for young children.
"We therefore concluded that the ad was inappropriately scheduled and an ex-kids restriction should have been applied to prevent the ad from being broadcast in or around children's programming."
Chanel was told the the advert must not appear again in its current form in or around programmes of particular appeal to children.