Adverts featuring supermodel Kate Moss with computer-enhanced eyelashes broke advertising rules, the Advertising Standards Authority (ASA) has said.
Rimmel denies Kate Moss wore false lashes during the photo shoot
The agency behind the Rimmel mascara campaign denied the model wore false eyelashes, but said they had been digitally "cleaned up and enhanced".
The watchdog ruled the 'Magnif'Eyes' mascara TV and magazine adverts broke rules by exaggerating product benefits.
Coty, which owns Rimmel, said it would continue to comply with ASA guidelines.
The advertising watchdog found the Rimmel magazine adverts were in breach of rules relating to truthfulness and substantiation, and the TV commercials, which claimed to give wearers "70% more lash lift", broke rules for misleading advertising and evidence.
It follows a similar ruling against L'Oreal in July when it was found to be in breach of rules after it said actress Penelope Cruz was wearing false eyelashes in its adverts.
The advertising agency responsible for the latest Moss campaign, J Walter Thompson, denied false lashes had been used for its photo shoot but it said her eyelashes had been "cleaned up and enhanced" in post-production.
However, because it did not provide evidence to prove false lashes were not used or reveal how much the appearance of Moss's eyelashes had been changed, the ASA concluded rules had been broken.
The ASA said TV claims of increased "lash lift" also misled viewers because it referred to a change in appearance and not in the resulting length of eyelashes.
Rimmel told the watchdog it had tested the enhancement claims on a panel of 10 women and digital images showed an average increase in "lash lift from root to tip" of 74.7%.
But the ASA banned the cosmetics company from repeating the adverts.
A disclaimer would also be required in any future adverts if a model's appearance was enhanced with post-production techniques or false eyelashes, it added.
David Allan, marketing director of Coty, which owns the Rimmel brand, said: "Coty had full approval from the Broadcast Advertising Clearance Centre for the Rimmel Magnif'Eyes advertising.
"This campaign ran in line with the original planned schedule. We recognise the ASA has become more stringent in their application and Coty will continue to be fully compliant with the ASA guidelines."