The ad depicted two women in a convertible sports car, set against a blurred background. It carried the copy "Faster is funner. Introducing the 2x faster New BT Yahoo Mail".
The two complainants argued that the ad was likely to cause harm because it "condoned and encouraged excess speed and irresponsible driving".
Yahoo defended the ad, saying it was unrelated to motoring and neither condoned nor encouraged speeding or irresponsible driving. The firm said the tagline referred purely to the speed of the mail service.
Yahoo said the car was not travelling over the speed limit and that no references were made to its actual speed. It claimed that the blurred background was intended to show the car was travelling on a mountain path. It said that several million people had viewed the ad and their customer care team had received no complaints.
The ASA, however, did not accept Yahoo's defence. While noting that Yahoo "intended the speed capabilities of Yahoo Mail to be the main focus of the ad", it decreed that "because the ad showed the car against a blurred background and was headlined "faster is funner", speed was the key message".
It concluded that the combination of headline and image "portrayed speed in a way that might encourage motorists to drive irresponsibly" and that it could be seen to "condone anti-social behaviour and irresponsible driving".
Yahoo was told that it cannot use the ad again in its current form and that it must not in the future "portray speed in a way that might encourage motorists to drive irresponsibly".
The ad has also drawn the ire of online commentators because of its bad grammar – in particular the use of the non-word "funner".