Two television commercials for Head & Shoulders shampoo have been criticised for implying that the products leave hair 100% dandruff-free.
The ASA wants the advertisements to be amended
The Advertising Standards Authority found against adverts for Ocean Spa and Cool Menthol varieties of the shampoo.
Procter & Gamble, the maker of Head & Shoulders, said its "100% of flakes" claim meant those visible to another person from a distance of two feet.
The ASA said the commercials would have to be amended before being re-shown.
In each advertisement, text appears at the bottom of the screen saying "100% visible flakes with regular use", to clarify the voice over.
Yet after investigating a complaint from a single viewer, the ASA agreed that Procter & Gamble could not back up the 100% claim for all users.
"We considered consumers were likely to interpret the claim '100% visible flakes with regular use' as referring to dandruff which was visible to them - eg when they were styling their hair in a mirror - as well as dandruff flakes visible to others," said the ASA.
"We did not consider that they would be aware that the claim had been based on flakes visible only to others from a distance of two feet."
Sticking by its two feet measurement, Procter & Gamble said this was a "realistic personal distance" for people speaking to each other.
It added that any closer than two feet could be seen as an invasion of personal space.
"The ASA ruling concerns a small point of clarification and we have already made the necessary adjustment to our advertising," said Procter & Gamble.
"Head & Shoulders has been a very popular anti-dandruff shampoo for many years with millions of satisfied consumers who love the product and remain loyal to Head & Shoulders as they know it effectively controls their dandruff."