The double-page promotion was extensively illustrated and carried the text "Win a boob job for your girlfriend worth £4,000. Make your lady a more rounded individual with our feel-good, selfless, world-first competition...
"Do you care enough about your beloved to bag her a prize that will transform her into a happier, more generous, intelligent, spiritual, interesting, witty, caring and politically aware version of the slightly second-rate person she is today? Do you want to win your lady a brand-new set of expertly crafted tits?"
There were three different arguments put forward by the complainants, and two were rejected by the Advertising Standards Authority.
However, the ASA upheld the argument that the ad was irresponsible because it could coerce women into having a serious surgical procedure that could cause physical and psychological damage.
Although Zoo said it was willing in future to make it clearer that the winner could spend the prize money on what they wanted, it was told not to repeat the approach.
The other two arguments were that the ad insulted women and the claim that the prize "would transform her" was misleading.
The ASA rejected these on the grounds that Zoo readers would not be offended nor interpret the claim literally.
Zoo's competition was last month criticised by the British Association of Aesthetic Plastic Surgeons as part of a backlash against what it called the "trivialisation of medical procedures" by cosmetic surgery companies and the media.
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