The ASA said the ad was misleading, ruling the Unilever brand did not have "sufficiently robust evidence" to support the claim that more people preferred Flora Butterly.
It launched an investigation after Lurpak manufacturer Arla Foods, and 30 viewers, complained.
In the ad, Rhodes conducts taste tests on shoppers using buttered crumpets.
When one woman is asked which batch she prefers she points to a selection which, Rhodes reveals, is spread with Flora Buttery.
The ad contains the line: "Out of 200 people tested 48% preferred Flora Buttery taste, 45% Lurpak Lighter Spreadable, 7% had no preference."
The ASA concluded that the 3% margin of people who preferred Flora Buttery to Lurpak Lighter, was not significant enough to warrant the claim.
It also ruled that 200 participants was not a large enough sample.
Clearcast, who qualified the ad to air, stated that although it noted the slim margin between the two products, it believed the on screen text made it clear consumers could make up their own minds.