The Comfort Pure campaign was created by Ogilvy & Mather and featured TV, outdoor and print ads.
The ad described the product as, "The UK's most trusted fabric conditioner for sensitive skin" and claimed it was hypoallergenic and dermatogically tested.
The TV ad's voiceover said "Comfort Pure, because nothing makes clothes softer".
Rival P&G, which owns detergent brand Ariel and fabric softener brand Lenor, complained about the use of the line "nothing makes clothes softer", saying it was misleading because it was based on sales data, qualified later in the ad with the line, "Based on IRI sales data 08/10 to 12/10".
Unilever said the sales data was an "appropriate" measure of consumer trust in fabric conditioner products and the ad's claim was further supported by Comfort Pure being "the biggest selling fabric conditioner in the UK" targeted at sensitive skin.
Unilever said: "If consumers did not trust a brand, particularly in the context of a product marketed as suitable for sensitive skin, they would not consistently return, in high numbers, and make repeat purchases."
The ASA said the use of sales figures in the ads contradicted rather than clarified the "most-trusted" brand claim and deemed the ads misleading.
It ruled the print and outdoor ads were misleading because they did not support the most-trusted claim with any evidence.
It banned the TV, outdoor and press ads from reappearing in their current form.