One person complained that the ad, which showed two women in bikinis, was offensive as it showed the younger of the two, who looked like a child, in a sexually provocative pose.
The advertiser, NH Hotels, said the ad was part of a campaign promoting holiday reservations and felt the ad should be viewed in the context of the overall campaign.
It added that the opinion that the girl was shown in a sexually provocative pose was subjective and most viewers would understand the ad to relate to booking a summer holiday.
Lastminute.com said the ad had appeared away from the main booking path but had been shown in error and once the site became aware of it, was removed immediately.
The ASA felt that because the ad appeared in isolation on Lastminute.com, it would not necessarily be viewed as part of a larger campaign. It felt the image could be seen to sexualise children and thus cause offence.
The ad must not appear again in its current form.