The watchdog responded to 22 complaints that suggested the TV ads were socially irresponsible.
Nasty Gal said that model was 178cm (5ft 11in) tall and weighed 134lbs (9st 8lb or 60.8kg) – which gave her a BMI of 18.8, within the healthy weight range according to the NHS.
But the ASA said that although the model appeared to be "in proportion" in most shots, there were some that drew attention to her slimness – while it flagged up a shot in which she sprays mist on herself, in which her rib cage is visible and prominent.
It ruled that in these sections, the model appeared to be unhealthily underweight, which meant the ads breached the BCAP code rules on social responsibility.
Nasty Gal said it had complied with the ruling, but was critical of the ASA's conclusion.
A spokeswoman said: "At Nasty Gal we want to do all we can to use our voice to promote body positivity and diversity. We have worked with the model in this ad many times and she is represented by one of the world’s leading agencies, whose key priority is the health of their models. Her athletic build is in accordance with the NHS guidelines for a healthy adult woman."
Most recently, online retailer Motel Rocks was ordered in January this year to remove four images from its website featuring a model the ASA determined to look unhealthily thin.