The Advertising Standards Authority (ASA) has issued guidance on the way women are portrayed in adverts.
It said it "regularly" receives complaints about the way in which women and, to a lesser extent, men are portrayed in ads, and that it "takes a firm line" on adverts "which irresponsibly sexualise or objectify people".
Most of the complaints relate to websites and outdoor adverts for 'fast fashion' brands.
Advertisers, especially those of lingerie and swimwear, are being told to take care to ensure that their adverts are not "overtly risqué" and that models are "not show in an overtly sexual way or objectifying way".
Adverts that display "gratuitous nudity when it is irrelevant to the product", or ads that "focus solely on the model's body" while obscuring their face "are likely to be a problem", the regulator said.
"Marketers should take care to ensure that the content of their ads is suitable for the media in which they will appear, especially if they are likely to be seen by children," the ASA said.
"The ASA takes a particularly strict approach against sexually suggestive imagery in outdoor advertising.
"In short, imagery considered 'overtly/explicitly sexual' is prohibited in outdoor, untargeted, media and material considered 'sexually suggestive' should carry a placement restriction, ensuring it does not appear within 100 metres of a school."